Add the ability to inject a custom argument into Epics about redux-most HOT 4 OPEN

redux-most offers 2 separate APIs: a redux-observable-like API, where Epics get passed an action stream & a store middleware object containing dispatch & getState methods, and a stricter, more declarative API, where Epics get passed an action stream & a state stream.

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Initially, redux-most offered the same API as redux-observable, where Epics received an action stream & a store middleware object containing dispatch & getState methods. However, it now offers both that API and another stricter, more declarative API which eliminates the use of dispatch & getState. The reason for this is that I rarely found myself using the imperative dispatch method. It's not really needed, because you can use switch, merge, mergeArray, etc. to send multiple actions through your outgoing stream. This is nice, because it allows you to stay locked into the declarative programming style the entire time.

However, using getState was still required in epics that needed access to the current state. I wanted a nice, convenient way to access the current state, just like I had for dispatching actions. So, I created an alternate API where Epics receive a stream of state changes rather than the { dispatch, getState } object. This state stream, combined with the new withState utility function, let's you use streams for both dispatching actions & accessing the current state, allowing you to stay focused & in the zone (the reactive programming mindset).