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.
Ah, thanks for the example— very helpful.

A better test would probably be to download a small example file from somewhere like modarchive.org, and attempt to load that. Is there a canonical way to grab test data in homebrew, with a specified SHA1, etc?