Write a simple parser to mangle pygmentize output to use email-friendly style attributes

Summary:
Ref T9790. This mangles pygmentize output so it does <span style="..."> instead of <span class="...">.

It doesn't use a regular expression, although it probably could.

I believe this is safe for all inputs, but it doesn't really matter: we'll only feed it safe inputs anyway, and it's only going to be used for email which we can't really do anything unsafe in.

Test Plan: Added unit tests; ran unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9790

Differential Revision: https://secure.phabricator.com/D15845