Adds 'demo_stomp' launch file (#82)

This commit adds the 'demo_stomp.launch' file that is referenced in
https://ros-planning.github.io/moveit_tutorials/doc/stomp_planner/stomp_planner_tutorial.html.
Fixes #44.

launch/demo_stomp.launch

+<launch>
+  <include file="$(dirname)/demo.launch">
+    <arg name="pipeline" value="stomp"/>
+  </include>
+</launch>
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