Add support for param:// to use ROS param

[sloretz]

From ros/robot_model#17 created by @eacousineau

It would be nice to have a param:// protocol as well, so that the contents of an URDF could be set to something like 'param:///robot_description' or 'package://my_robot/urdf/robot.urdf'.

If putting that in complicates things (since it requires the ROS be init'd), then there could be an extra boolean flag to make it explicit (and error out if param:// is requested without that flag).

If others are interested, I could implement that feature.

[DLu]

I have nothing against this feature, although there may be a couple corner cases where it acts weird. I think if you do implement it, definitely put the extra flag in there.