Improve the quality of parse/delimiter rules

[gjtorikian]

This PR does a few neat things:

• Every filter will go through the same "mega" function, *_do_filter.
• It adds options for toggling delimiters. (Closes https://github.com/gjtorikian/mtex2MML/issues/34.)
• It vastly improves the heuristic for $..$, based on the quote from @jgm: http://bit.ly/1HVtemx
• It fixes many compilation warnings.
• It renames the lexer/parser files to what they actually do.
• Runs make format to format the parser.y code