Support for react-router 4

[aaronjensen]

It's alpha right now so we may not want it here, but I'm opening this issue to at least share the definitions I have so far for 4.0.0-alpha.3

https://gist.github.com/aaronjensen/48fcfaa0d540805d30fa18114c224f3f

[jeffmo]

Nice. flow-typed is pretty strict about the versioning patterns that are allowed, and I think maj.min.ptc-alpha versions aren't currently supported yet -- but we probably could/should support that!

[jeffmo]

Linking this in to #392 to track support for prerelease identifiers

[kouak]

Now that react-router v4 has been released, and react-router v4 definitions were merged (53afdde), this issue could probably be closed.

ping @aaronjensen

[aaronjensen]

Works for me, if #714 is the only API change so far then awesome. @jeffmo if you could merge #714 in asap that'd be great, it's a breaking change for the release version.