Added support for text .fnt files.

I basically implemeneted a scanner (all on the stack, no heap) then
a very poor parser. A DFA for the scanner can be found in
`text-tokenizer.svg`.

I'd like to mindlessly shove everything into a hashmap instead of
populating this BMFont_ParseInfo struct because I anticipate it
would save a lot of code. However, I'm still working out the
details for that.