Add five new extensions to XML, YAML in support of ROS usage.

These extensions are in common use in packages part of ROS, the Robot Operating System.

[urdf](http://wiki.ros.org/urdf), [srdf](http://wiki.ros.org/srdf): These are Robot Description Files, XML documents which describe the physical realities of a robotics platform, for the purposes of consumption by common libraries.

[xacro](http://wiki.ros.org/xacro) is for input files to the XML macro processor xacro, which is used in ROS to output URDF and SRDF files.

[launch](http://wiki.ros.org/roslaunch/XML): Documents which describe sets of ROS nodes to launch together.

[rviz](https://github.com/ros-visualization/rviz/blob/hydro-devel/default.rviz): YAML configuration files belonging to the [rviz](http://wiki.ros.org/rviz#Overview) utility.

Each one has been in use for several years in various ROS-related applications; lots of examples should be apparent in orgs like ros, ros-drivers, ros-visualization, pr2, turtlebot, husky, etc.

Thanks for your consideration!
Merge pull request #791 from mikepurvis/patch-1

Add five new extensions to XML, YAML in support of ROS usage.