Support Promises

[Justinohallo]

The Redux Dispatch will return a promise, allowing you to chain actions after the dispatch has taken place.

While the native useReducer dispatch does not return a promise, would it make sense for this Thunk implementation to do so?

[nathanbuchar]

Wouldn't be a bad idea, I guess I haven't used Redux in a while so thunks returning promises is new to me. I'll be honest though this isn't high on my list, but I'd welcome a PR :)