Used config to identify valid exts; Made pre & nowiki case insensitive

* Used config information to identify valid extensions and removed
  temporary hack.

* Made nowiki and pre tag matching case-insensitive.
  - RTing of these in original case is not yet done -- this seems
    a little bit messy and will require fixing any tag-comparison
	that doesn't do normalized case comparison.  Will also require
	fixes to serializer.  Can be done post ongoing refactoring.

* 1 wt2wt test worse off -- this is because of RTing in normalized
  lower-case of nowiki tags.

Change-Id: Ie727b76926247d65abf4b9c3fe79afc32eb03827