External mappings for MIDI instruments

[Bubobubobubobubo]

This draft PR proposes the addition of custom external instrument MIDI mappings exposing new parameters to be patterned. A mapping currently
takes this form:

let mapping = {
  port: "MIDI Bus 1",
  dummy: {cc: 80, channel: 1}, // channel is optional
  controller: true // whether or not to emit a note
}

note("C E G C").dummy(0.2).midi(mapping)

It could be much better with the following additions:

• custom range mapping
• automatically declaring new params (if they do not exist)
• documentation