Add substitution nesting

Rationale:
 - Needed for supporting eg. comparison of params (see last comment in https://github.com/ros/ros_comm/pull/341#issuecomment-37585632)
 - It enables stuff like eg. &lt;include file="$(find $(arg robot)_description)/robots/upload_$(arg robot).launch" /&gt;.
 - Why shouldn't we have this? :)
It is already checked in the wrapper function _find_hack().