* (bug 5384) Fix <!-- comments --> in <ref> extension

* (bug 5384) Fix <!-- comments --> in <ref> extension
* Nesting of different tag extensions and comments should now work more
  consistently and more safely. A cleaner, one-pass tag strip lets the
  'outer' tag either take source (<nowiki>-style) or pass it down to
  further parsing (<ref>-style). There should no longer be surprise
  expansion of foreign extensions inside HTML output, or differences
  in behavior based on the order tags are loaded.