Add `event` as second parameter to listeners?

[ronen]

Hi, it would be convenient (for me :) if the listener for on and once were passed the event, i.e. have the signature be listener(data, event). This would:

• make it possible to for the listener to disambiguate the event when used with multiple events on([...events], listener)
• similarly be convenient to use the same listener in multiple separate calls to on or once with different events
• be non-breaking / backwards-compatible (unless there's some subtlety that I'm not thinking of?)

It looks like it'd be a fairly small change to the code plus a few extra tests.

An awkwardness is that the listener parameter order would be the opposite of onAny(listener), which could be confusing. One way around that could be to define a symbol such as Emittery.Any so that on(Emmittery.Any, listener) would be equivalent to onAny(listener), except using the signature of on listeners. Or instead of a symbol, could support on('*', listener) as a special case of possible future wildcard/namespace support (#31, #80) — but that would be a breaking change since '*' is currently a valid ordinary event name.

There are workarounds of course — include the event name in the data, or write the generic listener and curry the event name into event-specific listeners — but they're a little more cumbersome.

What do you think? If you like the idea I could put together a PR when I get a chance.

[ronen]

Actually, for full generality the signature could be listener(data, event, emitter) allowing the same handler to be used on multiple emitters.

[ronen]

Actually I realize probably better would be listener(data, { event, emitter }) so that you don't have to remember positional parameter order, and leaving the door open for future metadata.