Allow passing options to sub protocols

Before this commit we had an issue where configuring a
Websocket connection was simply not possible without
doing magic, adding callbacks or extra return values.
The init/2 function only allowed setting hibernate
and timeout options.

After this commit, when switching to a different
type of handler you can either return

  {module, Req, State}

or

  {module, Req, State, Opts}

where Opts is any value (as far as the sub protocol
interface is concerned) and is ultimately checked
by the custom handlers.

A large protocol like Websocket would accept only
a map there, with many different options, while a
small interface like loop handlers would allow
passing hibernate and nothing else.

For Websocket, hibernate must be set from the
websocket_init/1 callback, because init/2 executes
in a separate process.

Sub protocols now have two callbacks: one with the
Opts value, one without.

The loop handler code was largely reworked and
simplified. It does not need to manage a timeout
or read from the socket anymore, it's the job of
the protocol code. A lot of unnecessary stuff was
therefore removed.

Websocket compression must now be enabled from
the handler options instead of per listener. This
means that a project can have two separate Websocket
handlers with different options. Compression is
still disabled by default, and the idle_timeout
value was changed from inifnity to 60000 (60 seconds),
as that's safer and is also a good value for mobile
devices.

examples/eventsource/src/eventsource_handler.erl

@@ -11,7 +11,7 @@ init(Req0, Opts) ->
 		<<"content-type">> => <<"text/event-stream">>
 	}, Req0),
 	erlang:send_after(1000, self(), {message, "Tick"}),
-	{cowboy_loop, Req, Opts, 5000}.
+	{cowboy_loop, Req, Opts}.
 
 info({message, Msg}, Req, State) ->
 	cowboy_req:stream_body(["id: ", id(), "\ndata: ", Msg, "\n\n"], nofin, Req),