support array of URDF strings

[sloretz]

From @mikepurvis ros/robot_model#48

Would be great if either a) the node could be run multiple times (with a namespaced robot_description parameter), or b) the node could accept multiple strings and unify them to a single large model.

Previous external efforts:

Compose urdf by globbing together xacros into a file of include tags.

Compose urdf using yaml files defining known accessories and a GUI tool.

[peci1]

You can run the node for different parameters. The robot_description parameter name is actually remappable, so you can start the node as

rosrun robot_state_publisher robot_state_publisher robot_description:=my_robot_description.