Enable font selection via xetex

This commit makes font selection easy by switching the compiler to
xetex. When using xetex, fontenc, inputenc, lmodern, and fixltx2e are
all superseded by fontspec. Unfortunately, cmap is not xetex-compatible.
This does cause some minor issues when one searches for ligatured
characters (e.g. ff, fi), but it's possible to work around that by
deactivating those ligatures altogether. (Note also: For whatever
reason, the fontspec name for Latin Modern is actually "Computer
Modern".)