Add Flow-types

[Ephem]

The new functionality (located mainly at the bottom of ReactPartialRenderer) was written without Flow-types since this started as an exploration and I wasn't comfortable moving fast with Flow. Now that things are more stable it would be nice to add types.

Feel free to help out if you are comfortable with Flow and want to explore this project, if anyone wants to tackle this I would love to help out. I will get around to this myself eventually, but it's not at the top of my list.