Stop event propagation inside handler.

[doowb]

I'd like to be able to break out of the handler loop if a handler specifies that it has handled the event.
This PR lets a handler fn set this.handled = true that will break out of the emit loop.

emitter.on('login', function (user) {
  if (user.username === 'doowb' && user.password === 'password') {
    this.handled = true;
    this.user = user;
  }
});

emitter.on('login', function (user) {
  this.error = 'Invalid username and/or password';
});

emitter.emit('login', {username: 'doowb', password: 'invalid'});
console.log(emitter.error);
//=> Invalid username and/or password

emitter.emit('login', {username: 'doowb', password: 'password'});
console.log(emitter.user);
//=> {username: 'doowb', password: 'password'}

[vendethiel]

Seems like not too many people have needed this; so I'm going to close it.

Sorry for the delay.