feat: basic websocket support

[kvalev]

This PR adds a very very basic (and highly likely buggy) support for mocking websockets.

[kvalev]

@h2non the PR is missing a lot of tests, documentation and a bit of a polish, but let me know what you think so far.

[h2non]

It will not work. Mutex synchronization is required to make it safe. However, I feel that's another feature that has to be implemented in a separate PR.

[h2non]

LGTM! This looks promising and ambitious!
Adding some extra tests would be great to cover more complex scenarios.

It would be great it can support as well this client, which works on top of asyncio:
https://github.com/aaugustin/websockets

At user API design level, I wonder about how the mock DSL can be improved to mock up exchanged frames with arbitrary responses provided by a routine.

I guess one obvious solution would be allowing the user to pass a function with a signature like:

def framer(frame, request) -> MockFrame:
   // Do magic things
   return mock

At a user definition API level, I'm inclined to overload the body() method to support this case:

def frame_mock(frame, request) -> MockFrame:
   // Do magic things
   return mock

(pook.get('ws://some-non-existing.org')
    .reply(204)
    .body(frame_mock)

Alternatively, implement a custom method for frames specific case:

(pook.get('ws://some-non-existing.org')
    .reply(204)
    .frames(frames_mock)

Opinions?