Optionally don't put comments in generated files

[lucasw]

The <!-- replace won't work with multi-line comments, but package.xml.in hasn't any and hasn't changed in 3 years.

Example:

catkin_create_pkg --no_comments test
Created file test/package.xml
Created file test/CMakeLists.txt
Successfully created files in /home/lucasw/ros/test. Please adjust the values in package.xml.
cat test/package.xml
<?xml version="1.0"?>
<package format="2">
  <name>test</name>
  <version>0.0.0</version>
  <description>The test package</description>

  <maintainer email="lucasw@todo.todo">lucasw</maintainer>

  <license>TODO</license>

  <buildtool_depend>catkin</buildtool_depend>

  <export>

  </export>
</package>