Add chunked response body fun

Adds a new type of streaming response fun. It can be set in a similar
way to a streaming body fun with known length:

Req2 = cowboy_req:set_resp_body_fun(chunked, StreamFun, Req)

The fun, StreamFun, should accept a fun as its single argument. This
fun, ChunkFun, is used to send chunks of iodata:

ok = ChunkFun(IoData)

ChunkFun should not be called with an empty binary or iolist as this
will cause HTTP 1.1 clients to believe the stream is over. The final (0
length) chunk will be sent automatically - even if it has already been
sent - assuming no exception is raised.

Also note that the connection will close after the last chunk for HTTP
1.0 clients.