lib/off.js
function off (state, eventName, handler) {
  state.emitter.removeListener(eventName, handler)

  return state.api
}
function off (state, eventName, handler) {
  state.emitter.removeListener(eventName, handler)

  return state.api
}
function off ( state , eventName , handler ) { state . emitter . removeListener ( eventName , handler ) return state . api }
removes a listener for the specified event

It will unsubscribe at most, one instance of a listener for a particular event.
If any single listener has subcribed multiple times to the same event,
then `off` must be called multiple times.

Supported events:

- `add`
- `update`
- `remove`
- `change`

@param  {String} eventName   Name of event, one of listed above
@param  {Function} handler   callback for event
removes a listener for the specified event