game-music-emu 0.6.0 (new formula)

game-music-emu is a library that implements emulation for various video game music formats (NSF, SPC, VGM etc.). It's used in e.g. mpd. This PR adds a formula `game-music-emu` (as the [source project](https://code.google.com/p/game-music-emu/) is called), with an alias `libgme` (as [other](https://github.com/FFmpeg/FFmpeg/blob/master/configure#L218) [projects](http://git.musicpd.org/cgit/master/mpd.git/tree/configure.ac#n899) often refer to it).