Streaming messages

I've had the idea in my head for a while already that I would love to support sending a `Stream<Item = T> where T: Into<Payload>` as a binary or text message. The stream would be consumed by tokio-websockets and each item yielded by it would be sent as a single frame. Upon receiving `Ready(None)` we could send an empty frame with the final bit set to true. I imagine this would be fairly easy to add and probably a good alternative to exposing a raw API for sending frames.