Add support for a MIDI synthesizer

**Problem I'm having**
I have a bunch of MIDI files I'd like to play and it is semi-annoying to have to download them and then open VLC to play them.

**Describe the idea / solution you'd like**
I'd like to have support for a MIDI synthesizer that allows the client to specify a custom soundfont file as well as allow the server-owner to set a default.

**Describe any alternatives you've considered**
Auto trans-code any MIDI to OPUS or MP3 server-side and send that back to the client. (This would be a bad idea, because --as far as I know-- MIDI has to be recorded back in real-time i.e a 1-hour MIDI would take 1 hour to trans-code.)