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.
For debugging, you can `system "ls -R"` inside the test and run brew test with -v to see where the resource ends up; I think it'll be named from the download URL. The argument stage takes is a pathname, not a filename.