Lack of ws.Message (un)marshaller

I'd like to hear opinions on the standard inclusion of a `FromWSMessageUnmarshaller` for `ws.Message`. This would reduce standard boilerplate when dealing with web sockets. I'd even go as far as suggest that implementations for JSON and XML be included. Whenever I'm dealing with web sockets, most, if not all, communication is done using a JSON transport. I've got the implementations mostly ready, so I can produce a PR if necessary.

No file or test is named. Start by reviewing ws.Message and the existing unmarshaller APIs, then assess the proposed FromWSMessageUnmarshaller for JSON and XML WebSocket communication. Done requires an agreed API and a decision on whether the standard inclusion should proceed.

Written by the indexing model from the issue text.