Also update the CHANGELOG and copyright years.

This new protocol option is a fun.

It expects 3 args: the Status code used in the reply (this is the
cowboy_http:status() type, it can be an integer or a binary), the
headers that will be sent in the reply, and the Req. It should
only return a possibly modified Req. This can be used for many
things like error logging or custom error pages.

If a reply is sent inside the hook, then Cowboy will discard the
reply initially sent. Extra caution must be used in the handlers
making use of inline chunked replies as they will throw an error.

This fun cannot be used as a filter, you can either observe the
reply sent or discard it to send a different one instead.

The hook will not be called for replies sent from inside the hook.

Introduces 3 low level functions and updates the existing higher
levels functions. The new primitives are has_body/1, body_length/1
and stream_body/1. In addition to that, a helper function
init_stream/4 has been added.

Streaming a body implies to decode the Transfer-Encoding and
Content-Encoding used for the body. By default, Cowboy will try
to figure out what was used and decode them properly. You can
override this if you want to disable this behavior or simply
support more encodings by calling the init_stream/4 function
before you start streaming the body.

Thanks go to @superbobry for pointing it out.

At the same time rename http_headers/0 to cowboy_http:headers/0.