XML Formatting and Linting

Right now, XML linting is accomplished by running each launch file through xmllint's formatting and running a diff to compare with the original file. This results in a very strict set of style constraints:

• Format using 2 spaces for indentation.
• Use double-quotes " instead of single-quotes '.
• No space before each closing tag (e.g. name="value"/> instead of name="value" />.
• Blank lines are allowed, but lines containing only whitespace are not. Likewise, lines cannot end with whitespace.
• Comments are not allowed after a tag. Instead of

  <tag> <!-- comment -->

  use

  <!-- comment -->
  <tag>

• Tags cannot span multiple lines. If a tag is getting too long, consider using <arg> tags, e.g. instead of

  <tag attr1="A very very very very very long string" attr2="Another very very very very very long string">

  use

  <arg name="attr1" value="A very very very very very long string"/>
  <arg name="attr2" value="Another very very very very very long string"/>
  <tag attr1="$(arg attr1)" attr2="$(arg attr2)">