functionaliy to ignore content in tags

As discussed, functionality has been added here that does the following:

 1. Ignores content between ``<pre>`` and ``<code>`` tags
 2. Allows the user to specify additional tags where content can be ignored

Some things to note:
 * I removed the ``<pre>`` and ``<code>`` tags from line 151 in filters.py which used them to ignore processing within the caps filter. Seeing as these tags are now not processed by default, it is redundant to leave them in.
 * I also removed ``<math>`` tag from the same list. The functionality added in this revision should allow one to ignore ``<math>`` explicitly.

Please review and ask me to fix anything if it is needed. Otherwise can this be incorporated as soon as possible.