Add Flow support

[soulofmischief]

I'd like to see Mithril added to flow-typed.

We could probably just run the TS definitions through flowgen and then clean it up from there.

[charlag]

I want to point out that besides generating TS types we have working declarations at Tutanota: https://github.com/tutao/tutanota/blob/master/flow/libs.js#L16 and we would be willing to upstream them It's a little bit of a mess right now with multiple definitions overlapping and spread across the file but it could serve as base.

Definitions for stream and lifecycle are below.

[dead-claudia]

PRs for Flow type support will be accepted as long as they work. I do know enough of Flow's type system to get by, but I'm not nearly as knowledgeable of it as I am with TS, so I could review a patch to ensure accuracy, but writing them would be a bit of a struggle.