feat(urdf.py): show URDF object continuously

[lixinyao11]

modified the show function in class URDF, enabling continuous calling robot.show()

Example:

>>> from urdfpy import URDF
>>> robot = URDF.load('tests/data/ur5/ur5.urdf')
>>> robot.show(cfg={
...  'shoulder_lift_joint': -2.0,
...     'elbow_joint': 2.0
... })
>>> robot.show(cfg={
...  'shoulder_lift_joint': 1.0,
...     'elbow_joint': 2.0
... })
>>> robot.show(cfg={
...     'elbow_joint': 2.0
... })

The visualization window will appear when show() is called for the first time. All the following calls will be showed in the same window, only updating the robot.