SocketManager: Add optional argument pass-through to socketio.AsyncServer constructor

[inactivist]

There are times when it may be necessary to be able to pass optional arguments to the AsyncServer constructor.

The current SocketManager constructor passes a fixed set of arguments when it initializes the _sio instance.

Perhaps an optional dict argument, socketio_server_args could be passed down into the AsyncServer constructor, as kwargs.

I'll try to work up a PR for this soon.

[pyropy]

I added async_mode argument that defaults to asgi. Take a look at #21 . Sorry because this took this long..

Let me know if that resolves your issue 😄