Hello 2.0.0!

This should work very similar to normal supervisors,
in particular during the shutdown sequence when the
connection process goes down or switches to Websocket.

Processes that need to enforce the shutdown timeout
will be required to trap exits, just like in a supervisor.
In a vanilla Cowboy, this only matters at connection
shutdown, as Cowboy will otherwise wait for the request
process to be down before stopping the stream.

Tests are currently missing.

Existing tests pass. A number of things remain to be done.
Has only been tested with Gun so far. Feedback welcome!