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 perfect. Have updated my alternative branch, it now works: https://github.com/mikepurvis/homebrew/blob/add-libmodplug-bettertest/Library/Formula/libmodplug.rb

Not sure if this is wanted on the main PR, though— it does introduce a test dependency on an external webserver. Let me know if it's desired and I'll update the PR accordingly.