Additional markup formats: AsciiDoc, reStructuredText

[apjanke]

Would be nice to support some additional markup formats as output targets, including AsciiDoc and reStructuredText.

This probably requires reworking the entire processing pipeline to convert the LaTeX to an abstract data structure, and then have converters for that data structure to each output format target, instead of just doing a bunch of regexpreps on the input latex.

Or maybe we can live with just managing multiple latex-munging implementations for the different formats. That might be easier, if less "elegant".