Use html5lib to parse HTML instead of using regexs

HTML is not a regular language and thus cannot be accurately parsed
with a regex. The regexs that pip had were gnarly, and required
multiple levels of them in order to "work".

Using html5lib instead of regexs makes the parsing step simpler,
better tested, and much more robust.

There is still some use of regex to parse html. Namely the hack
that looks for a <th> tag with a "rel" after it. This hack is
required to work inside of HTML comments so it cannot be parsed
as HTML.