Yup: Add Flow types

[lfantone]

I started to write the _flow types_ for yup. It will be nice if someone could validate them or also add the missing ones:

https://gist.github.com/lfantone/e73634b28eae1489757f493961e12961

One this one is done, we can then create a PR to flow-typed

[baransu]

It would be great to ship flow types in library itself rather than in flow-types if possible 🙂

[jquense]

I don't want to maintain types but i'm happy if someone wnats to post them on flow-typed. I realize that's less idea than here but i don't have the time or patience to maintain them myself sorry!