add meta-tag to roslaunch file

[Kei Okada]

As reporeted on https://code.ros.org/trac/ros/ticket/3515, we'd like to propose adding custom meta-tag to roslaunch file.

One example is to add reStructuredText documentation into a launch file. currently we added <sphinxdoc> tag use another program to read and generate rst file.
source : https://jsk-ros-pkg.svn.sourceforge.net/svnroot/jsk-ros-pkg/trunk/openrave_planning/orrosplanning/launch/collada_rviz_display.launch
output : http://ros.org/doc/api/openrave_robot_control/html/
Currently, we always get "WARNING: unrecognized tag sphinxdoc" when launching the file and this annoying users

Another example is to support custom tag to control the user applications. For example,
http://code.google.com/p/rtm-ros-robotics/source/browse/trunk/rtmros_common/openrtm/launch/MobileRobot_2DSimulator_Example.launch
uses <rtconnect> <rtactivate> tag and rtmlaunch.py script read this xml file and extract custom tag and do some jobs.

I'm not sure about roslaunch implementation, but extending roslaunch syntax with plugin-in style seems nice,
so adding
<custom-tag name="shpinxdoc" script="" />
<custom-tag name="rtconnect" script="/foo/bar/rtconnect.py" />
int roslaunch
OR
support "rospack roslaunch" as "rospack langs" and if system found <custom-roslaunch-package>/launch/rtconnect.py, then
check rtconnect tag and invoke rtconnect.py script.
???