What are the constrains on the slice passed to connectionFromArraySlice?

Hello guys!

It seems to me than to pass a valid slice to _connectionFromArraySlice_ you basically need to know the start offset and end offset to make the slice bigger. Do you agree?

If yes, I'm happy to extract [this piece](https://github.com/graphql/graphql-relay-js/blob/84a126866a01f78513bc3bf32413aaaae83e7931/src/connection/arrayconnection.js#L71-L98) into its own function and add tests :)

Cheers