MIDI output to non-Java device

[daveyarwood]

Moved from #259.

Quoting @0atman:

While there are some excellent soundfonts out there, and thanks to this project I am now aware of them, nothing can beat using your own DAW, VSTs or physical devices for output.

This feature would also give us MIDI file saving, in a round about way ("record" alda into any DAW, then save the midi to file from there)

Quoting myself:

Great idea! I think this could potentially be a very easy feature to implement.

We are already firing MIDI note on/off events (as well as volume and panning messages and assigning instrument patches to channels) in real time, using our own scheduler. We're just sending these MIDI messages to a JVM MIDI synthesizer instance, instead of emitting them in a way that they could be used by another program.

We could add an option that would tell Alda to just emit the messages instead of sending them to the JVM MIDI synth.

See the original issue for some implementation notes.