[feature request] multiple callbacks for an event

[Andarist]

At the moment it's only possible to setup a single callback for a given event type -

postmate/src/postmate.js

Line 156 in 8277fc3

this.events[eventName] = callback

I think it's really convenient to hook into a single event from multiple places (therefore with multiple callbacks). Also it would be cool to have corresponding off method, but that's a separate issue.

If you think that would be a nice addition to the library I can add it quickly, I would just use extremely small mitt (<200 B) as underlaying emitter.

[yowainwright]

@Andarist I'd love to see the PR! Thanks!!!