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.
Yes, you can stage a resource from the test block, which looks like this: https://github.com/tdsmith/homebrew-testtap/commit/646dfaad377efffc4989a24a260d7c8bc0fdec07#diff-6739ec5481e1a452bc8cbaab512ca201