More development work

Among other things, added handling for:
* Math (Latex, MathML, and regular mathphrase equations)
* Misc elements like link, lineannotation, phrase, footnoteref, informalfigure, prefaceinfo, chapterinfo, etc.
* A bunch of more obscure inline elements like guibutton, symbol, etc.
* Code block language attribute
* Processing instructions
* Parametrized xref labels