v6.1.0: Control state reducer and dispatcher

This adds two new props/options: reducer and dispatcher. These wrap the internal reducer and dispatcher to allow full control over React Async's internal state management. It also makes invocation of the promiseFn / deferFn part of the dispatched action so it can be delayed, cached or scheduled.

See https://github.com/ghengeveld/react-async/blob/master/README.md#reducer for details.