Revisit .off() and .clear()

[novemberborn]

Fixes #22. on(), onAny(), off() and offAny() now must be called with a listener function. You can no longer remove all listeners for a particular event. Renamed clear() to clearListeners() which is, well, clearer if Emittery is subclassed.

I discussed more advanced "disposal" behaviors in #22 but I think this is sufficient.

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PR against #24 as to avoid having to resolve the inevitable merge conflicts.

[novemberborn]

I realized clearListeners(eventName) would still work quite nicely, so I've returned that behavior. It's now similar to off(eventName), but without the "oops my listener variable was undefined" footgun.

Note that you can't clear just the onAny() events. We could have another clearAnyListeners() function but I doubt it'll be necessary.

[sindresorhus]

Great! This is how I should have done it from the start :)