Improve createReducer

[Mistereo]

This change allows us to write reducers which depend on additional data as described here: reduxjs/redux#749 (comment)

[dvdzkwsk]

Interesting. Would you mind adding a comment to link to that discussion above createReducer so people looking at the function understand the purpose of ...rest?

[Mistereo]

I've added comment, let me know if I need to correct the text (I'm not a native speaker).
I also wonder why we pass to reducer only payload from action?
If it's FSA I think there can be cases when we want to access meta or/and error properties.

[dvdzkwsk]

@Mistereo it was originally just a utility for my own use, and I only used it when it made sense. That said, I'll probably be changing it so it passes the entire action rather than just the payload.