Add formula for libmodplug.

This is especially for locally building the [OpenJazz](http://www.alister.eu/jazz/oj/) ([github](https://github.com/AlisterT/openjazz)) project on a Mac, but is generally interesting and useful to enthusiasts of chiptunes and tracker files.
Okay, I can't quite get the resource based test working— I don't think I'm getting filename/staging part done correctly. My WIP is here: https://github.com/mikepurvis/homebrew/commit/002f458624fb4b62deecc721c914fa715280f971

@mikemcquaid Would you like this included, or is it over the top?