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.
Can this test be modified to do something more substantial then e.g. `--version` or `--help`? See `cmake.rb` for an example of a formula with a good test. Thanks!