Copy ping payload data when creating a pong response

According to the RFC6455 spec section 5.5.3:

> A Pong frame sent in response to a Ping frame must have identical
> "Application data" as found in the message body of the Ping frame
> being replied to.

I've updated websocket_parser to allow ping and pong messages with arbitrary payloads and to provide the payload as a parameter to the on_ping and on_pong callbacks.

This PR updates Reel to use this feature in the latest version of websocket_parser.