Feature: Lazy emitter initialization

[hoichi]

At the moment fromEvent attaches its listener to pre-existing EventEmitter. And since it doesn’t start listening the observable is run, it’s quite possible for it to miss some events, which might be unwelcome in some cases.

Proposal: instead of an emitter, accept a callback that returns an emitter, and call it inside .run.

I think I’d rather create a separate lazyFromEvent than overload fromEvent and have an overhead of runtime parameter checking.