::: Protocol upgrades

Cowboy features many different handlers, each for different purposes.
All handlers have a common entry point: the `init/3` function.

The default handler type is the simple HTTP handler.

To switch to a different protocol, you must perform a protocol
upgrade. This is what is done for Websocket and REST and is
explained in details in the respective chapters.

You can also create your own protocol on top of Cowboy and use
the protocol upgrade mechanism to switch to it.

For example, if you create the `my_protocol` module implementing
the `cowboy_sub_protocol` behavior, then you can upgrade to it
by simply returning the module name from `init/3`.

``` erlang
init(_, _, _Opts) ->
    {upgrade, protocol, my_protocol}.
```

The `cowboy_sub_protocol` behavior only requires one callback,
`upgrade/4`. It receives the Req object, the middleware environment,
and the handler and options for this request. This is the same
module as the `init/3` function and the same options that were
passed to it.

``` erlang
upgrade(Req, Env, Handler, HandlerOpts) ->
    %% ...
```

This callback is expected to behave like a middleware. Please
see the corresponding chapter for more information.