Added Jason Blevins' mdwn_itex.pm plugin.

From
  http://jblevins.org/git/ikiwiki/plugins.git/plain/mdwn_itex.pm
Updated mdwn_itex.pm for Ikiwiki 3.00.
Document mdwn_itex plugin.
Fix name in template directive for mdwn_itex plugin.
Convert blog post URL for mdwn_itex plugin from physics.drexel.edu to tremily.us.