Ignore text based on user-specified regex

This functionality is needed for the Pelican blog project. Currently, Typogrify will clash with Latex math typesetting. This pull request gives Typogrify the ability to ignore certain things in the text by using a regular expression. If a regular expression is given as the second argument to Typogrify, then anything in the text that matches the regular expression will not be processed by Typogrify.

The main reason for doing this is due to Latex typesetting (Latex cannot have & altered to &amp - it will destroy the typesetting). But the pull request has been designed to be more general than this by using a regular expression.