CoreMIDI Channels for Go

#What The beginnings of a Go library to use CoreMIDI on OSX via Go channels.

This is super early right now (only note on/note off) so you should probably try to use youpy's go-coremidi instead.

#Why go-coremidi looks pretty good but its approach to output was not immediately clear. Further, my heavy use of actors for other stuff makes me think of channels as a decent abstraction for dealing with MIDI input and output.

I've got the rough beginnings of a sequencer coming together on top of this for sharing later.

#Next Will tackle sysex and input as it's necessary for my use. As noted above, probably best you use go-coremidi instead.