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.
Unfortunately, it's just a dylib and four headers— there's no binary to run, and the upstream source doesn't include tests, so they'd have to be patched in (or submitted upstream).

Thoughts?