Don't load `fontspec` unless necessary

This should fix the font issues on Xe/LuaTeX. Some new semantics are
introduced: `word` is now the default font stack on XeLaTeX, and
`fontspec` is only loaded when using `word` or `none` (but only when
also using Xe/LuaTeX). See documentation (font section) for more
details on this.

Closes #29.