Add Flow type definitions

[joshburgess]

Add Flow type definitions to the library

Look at the TypeScript type definitions for reference

[joshburgess]

Started working on this, but running into some issues as I don't think Most currently offers Flow definitions. It's on hold right now in the add-flow branch.

Also, if we change what arguments can be passed into Epics, we should update the TypeScript definitions before continuing with adding Flow to the library.

[joshburgess]

Update: Most 2.0 will feature both TypeScript & Flow definitions. 🎊 🎉

So, this is being put on hold until Most 2.0 is released. I will address it when upgrading redux-most to use Most 2.0. 👍

[joshburgess]

Looks like someone has added Flow definitions to the current version of Most (pre-2.0). So, I'll be looking at this soon.