Add Flow type definitions

[nrabinowitz]

Need to research best practices here - could export from the lib, or add directly to the flow-typed repo.

[bsr203]

typescript too please :-)

[nrabinowitz]

Now that we have TypeScript generated from JSDocs, we might be able to get Flow for free via https://github.com/joarwilk/flowgen

JSDocs > TS > Flow... What could possibly go wrong? :)