Enable font selection via xetex

This pull request implements font selection via the xetex compilation engine. xetex is included in both MikTeX and TeX Live, so there shouldn't be any issues in terms of getting running with xetex (relative to pdflatex, anyway). 

Unfortunately, cmap is not xetex-compatible. The PDFs are still searchable, but you'll see some minor issues when one searches for ligatured characters (e.g. ff, fi). That said, it's possible to work around the ligature issues by deactivating those ligatures altogether. 

(Note also: For whatever reason, fontspec's "Computer Modern" is the same as pdflatex's "Latin Modern")