Contrib library: D Port of sfMod

I've been working on porting some useful SFML extensions to D so they can work with DSFML, and I've got a working implementation of sfMod. It's not a wrapper--sfMod is a fairly light glue layer between the public domain libmodplug and SFML, and I've tried to make pretty much the same thing, but in D. sfMidi, by the same author, has similar functionality, that I'm going to be trying to adapt in a similar way, hence the grouping under "dsftrackmusic"

Since this inherits from audio.SoundStream, it'll require reworking if DSFML switches to an extern(C++) based API, but I'll happily do that if necessary.