Description Handlebar Adjusting Mechanism for Scooter Technical Field of the Invention The present invention relates to a scooter, in particular to a handlebar adjusting mechanism for a scooter. Background of the Invention Electric scooters may provide convenience for those who are unable to more freely. To satisfy the use of scooters by users having different heights and different arm lengths, it is necessary to design an adjusting mechanism on a handlebar to adjust the height position and the front-rear position of the handlebar. To satisfy the use of scooters in dark, it is necessary to mount a headlight on the handlebar of a scooter. However, an existing handlebar adjusting mechanism can not ensure the irradiation direction of the headlight mounted on the handlebar while adjusting the height position and the front-rear position of the handlebar. Summary of the Invention An object of the present invention is to provide a handlebar adjusting mechanism for a scooter, which may realize the adjustment of the height position and the front-rear position of the handlebar and ensure the irradiation direction of the headlight, thereby solving the problem that an existing handlebar adjusting mechanism for a scooter can not ensure the irradiation direction of the headlight mounted on the handlebar while adjusting the height position and the front-rear position of the handlebar. The technical purpose of the present invention is mainly solved by the following technical solutions: a handlebar adjusting mechanism for a scooter is provided, including a lower pedestal, an upper pedestal, a first connecting rod and a second connecting rod, characterized in that the lower pedestal is fixedly 1 connected to a front-wheel steering system of a scooter, while the upper pedestal is fixedly mounted with a headlight and fixed to a handlebar; given that the head direction of the scooter is a front direction and the tail direction of the scooter is a rear direction, a first upper pivot hole and a second upper pivot hole arranged in tandem at interval are provided on the upper pedestal, a first lower pivot hole and a second pivot hole arranged in tandem at interval are provided on the lower pedestal, a front-rear distance L between a center of the first upper pivot hole and a center of the second upper pivot hole is equal to a front-rear distance M between a center of the first lower pivot hole and a center of the second lower pivot hole, the position of the center of the first upper pivot hole, the position of the center of the second upper pivot hole, the position of the center of the first lower pivot hole and the position of the center of the second lower pivot hole correspond to four vertexes of a parallelogram, and the plane of the parallelogram is a vertical plane parallel to a front-rear direction of the scooter; an upper end of the first connecting rod is pivoted to the first upper pivot hole while a lower end thereof is pivoted to the first lower pivot hole, an upper end of the second connecting rod is pivoted to the second upper pivot hole while a lower end thereof is pivoted to the second lower pivot hole, and the length of a connecting line between the upper end of the first connecting rod and the lower end of the first connecting row is equal to that of a connecting line between the upper end of the second connecting rod and the lower end of the second connecting rod. In the present invention, as the position of the center of the first upper pivot hole, the position of the center of the second upper pivot hole, the position of the center of the first lower pivot hole and the position of the center of the second lower pivot hole correspond to four vertexes of a parallelogram and the upper pedestal and the lower pedestal are connected to each other through the first connecting rod and the second connecting rod, by adjusting the angle of front-rear inclination of the first connecting rod and the second connecting rod, the parallelogram may be deformed, so that the positions of the upper pedestal and the lower pedestal 2 change relatively, but the upper pedestal and the lower pedestal are always parallel to each other. And, as the lower pedestal is fixedly connected to the front-wheel steering system of the scooter and the position of the lower pedestal remains unchanged, the position of the upper pedestal changes relatively to that of the lower pedestal, so that the height position and the front-rear position of the upper pedestal are changed, and the adjustment of the height position and the front-rear position of the handlebar connected to the upper pedestal is thus realized. Furthermore, as the upper pedestal and the lower pedestal are always parallel to each other, no manner how the height position and the front-rear position of the upper pedestal change, the irradiation angle of the headlight mounted on the upper pedestal will not change. In this way, both the irradiation direction and irradiation effect of the headlight may be ensured. Preferably, the first connecting rod comprises an upper half of the first connecting rod and a lower half of the second connecting rod, the second connecting rod is a straight connecting rod, and an included angle is formed at a joint of the upper half of the first connecting rod and the lower half of the second connecting rod; and, a cylinder is mounted between the upper half of the first connecting rod and the lower pedestal, a mounting end of the cylinder being hinged to the lower pedestal, a piston rod of the cylinder being hinged to the upper half of the first connecting rod. By forming the first connecting rod from an upper half and a lower half with an included angle therebetween, a space for mounting the cylinder may be formed between the first connecting rod and the second connecting rod, so that the cylinder is mounted between the first connecting rod and the second connecting rod, and the adjusting mechanism becomes more compact in structure; meanwhile, the cylinder pushes the first connecting rod to move so as to change the angle of inclination of the first connecting rod and the second connecting rod and thus to realize the adjustment of the height position and the front-rear position of the scooter handlebar, so that it is more labor-saving to adjust the height position 3 and the front-rear position of the handlebar. The cylinder may be replaced by an electric push rod, a hydraulic cylinder or another device supplying a linear driving force. Preferably, the included angle formed at the joint of the upper half of the first connecting rod and the lower half of the second connecting rod is an obtuse angle open to the rear direction of the scooter. Hence, the spacing between the first connecting rod and the second connecting rod may be reasonably controlled, and the adjusting mechanism becomes more compact in structure. Preferably, the length of the upper half of the first connecting rod is larger than that of the lower half of the second connecting rod, and the length of the upper half of the first connecting rod is 2 to 4 times of that of the lower half of the second connecting rod. Further, the connection position of the piston rod of the cylinder and the upper half of the first connecting rod may be lowered, the overall length of the cylinder is thus shortened, and the adjusting mechanism becomes more compact in structure. Preferably, both the first connecting rod and the second connecting rod are straight connecting rods, and the length of the first connecting rod is equal to that of the second connecting rod. Therefore, the present invention is compact in structure, ensures the irradiation direction of the headlight mounted on the handlebar while adjusting the height position and the front-rear position of the handlebar, and is labor-saving in operations. Brief Description of the Drawings Fig. 1 is a state structure diagram of Embodiment 1 of the present invention; Fig. 2 is another state structure diagram of Embodiment 1 of the present invention; Fig. 3 is a state structure diagram of Embodiment 2 of the present 4 invention; and Fig. 4 is another state structure diagram of Embodiment 2 of the present invention. Detailed Description of the Invention The technical solutions of the present invention will be further described specifically as below with reference to the accompanying drawings by embodiments. Embodiment 1 As shown in Fig. 1 and Fig. 2, a handlebar adjusting mechanism for a scooter includes a lower pedestal 1, an upper pedestal 2, a first connecting rod 3 and a second connecting rod 4. The lower pedestal 1 is fixedly connected to a front-wheel steering system 5 of a scooter, while the upper pedestal 2 is fixedly mounted with a headlight 6 and fixed to a handlebar (not shown). Given that the head direction of the scooter is a front direction and the tail direction of the scooter is a rear direction, a first upper pivot hole 7 and a second upper pivot hole 8 arranged in tandem at interval are provided on the upper pedestal 2, a first lower pivot hole 9 and a second pivot hole 10 arranged in tandem at interval are provided on the lower pedestal 1, a front-rear distance L between a center of the first upper pivot hole 7 and a center of the second upper pivot hole 8 is equal to a front-rear distance M between a center of the first lower pivot hole 9 and a center of the second lower pivot hole 10, the position of the center of the first upper pivot hole 7, the position of the center of the second upper pivot hole 8, the position of the center of the first lower pivot hole 9 and the position of the center of the second lower pivot hole 10 correspond to four vertexes of a parallelogram, and the plane of the parallelogram is a vertical plane parallel to a front-rear direction of the scooter. An upper end of the first connecting rod 3 is pivoted to the first upper pivot hole 7 while a lower end thereof is pivoted to the first lower pivot hole 9, un upper end of the second connecting rod 4 is pivoted to the second upper pivot hole 8 while a lower end 5 thereof is pivoted to the second lower pivot hole 10, the length of a connecting line H between the upper end of the first connecting rod and the lower end of the first connecting row is equal to that of a connecting line S between the upper end of the second connecting rod and the lower end of the second connecting rod. The first connecting rod 3 includes an upper half of the first connecting rod 11 and a lower half of the second connecting rod 12, the second connecting rod 4 is a straight connecting rod, and an included angle is formed at a joint of the upper half of the first connecting rod 11 and the lower half of the second connecting rod 12; and a cylinder 13 is mounted between the upper half of the first connecting rod 11 and the lower pedestal 1, a mounting end of the cylinder 13 is hinged to the lower pedestal 1, a piston rod 14 of the cylinder 13 being hinged to the upper half of the first connecting rod 11. The length of the upper half of the first connecting rod 11 is larger than that of the lower half of the second connecting rod 12, and the length of the upper half of the first connecting rod is 3 times of that of the lower half of the second connecting rod. As shown in Fig. 1 and Fig. 2, the cylinder 13 pushes the first connecting rod 13 to move to change the angle of inclination of the first connecting rod 3 and the second connecting rod 4. By adjusting the angle of front-rear inclination of the first connecting rod 3 and the second connecting rod 4, the parallelogram may be deformed, so that the positions of the upper pedestal 2 and the lower pedestal 1 change relatively, but the upper pedestal 2 and the lower pedestal 1 are always parallel to each other. And, as the lower pedestal 1 is fixedly connected to the front-wheel steering system 5 of the scooter and the position of the lower pedestal 1 remains unchanged, the position of the upper pedestal changes relatively to that of the lower pedestal, so that the height position and the front-rear position of the upper pedestal 2 are changed, and the adjustment of the height position and the front-rear position of the handlebar connected to the upper pedestal is thus realized. Furthermore, as the upper pedestal 2 and the lower pedestal 1 are always parallel to each other, 6 no manner how the height position and the front-rear position of the upper pedestal 2 change, the irradiation angle of the headlight 6 mounted on the upper pedestal will not change. In this way, both the irradiation direction of the headlight 6 always along an arrow A and the irradiation effect of the headlight 6 may be ensured. Embodiment 2 As shown in Fig. 3 and Fig. 4, a handlebar adjusting mechanism for a scooter includes a lower pedestal 1, an upper pedestal 2, a first connecting rod 3 and a second connecting rod 4, both the first connecting rod 3 and the second connecting rod 4 being straight connecting rods. The lower pedestal 1 is fixedly connected to a front-wheel steering system 5 of a scooter, while the upper pedestal 2 is fixedly mounted with a headlight 6 and fixed to a handlebar (not shown). Given that the head direction of the scooter is a front direction and the tail direction of the scooter is a rear direction, a first upper pivot hole 7 and a second upper pivot hole 8 arranged in tandem at interval are provided on the upper pedestal 2, a first lower pivot hole 9 and a second pivot hole 10 arranged in tandem at interval are provided on the lower pedestal 1, a front-rear distance L between a center of the first upper pivot hole 7 and a center of the second upper pivot hole 8 is equal to a front-rear distance M between a center of the first lower pivot hole 9 and a center of the second lower pivot hole 10, the position of the center of the first upper pivot hole 7, the position of the center of the second upper pivot hole 8, the position of the center of the first lower pivot hole 9 and the position of the center of the second lower pivot hole 10 correspond to four vertexes of a parallelogram, and the plane of the parallelogram is a vertical plane parallel to a front-rear direction of the scooter. An upper end of the first connecting rod 3 is pivoted to the first upper pivot hole 7 while a lower end thereof is pivoted to the first lower pivot hole 9, un upper end of the second connecting rod 4 is pivoted to the second upper pivot hole 8 while a lower end thereof is pivoted to the second lower pivot hole 10, the length of a connecting line H between the upper end of the 7 first connecting rod and the lower end of the first connecting row is equal to that of a connecting line S between the upper end of the second connecting rod and the lower end of the second connecting rod. By adjusting the angle of front-rear inclination of the first connecting rod 3 and the second connecting rod 4, the parallelogram may be deformed, so that the positions of the upper pedestal 2 and the lower pedestal 1 change relatively, but the upper pedestal 2 and the lower pedestal 1 are always parallel to each other. And, as the lower pedestal 1 is fixedly connected to the front-wheel steering system 5 of the scooter and the position of the lower pedestal 1 remains unchanged, the position of the upper pedestal changes relatively to that of the lower pedestal, so that the height position and the front-rear position of the upper pedestal 2 are changed, and the adjustment of the height position and the front-rear position of the handlebar connected to the upper pedestal is thus realized. Furthermore, as the upper pedestal 2 and the lower pedestal 1 are always parallel to each other, no manner how the height position and the front-rear position of the upper pedestal 2 change, the irradiation angle of the headlight 6 mounted on the upper pedestal will not change. In this way, both the irradiation direction of the headlight 6 always along an arrow A and the irradiation effect of the headlight 6 may be ensured. 8