Proxy Protocol support

[ycytt-rs]

Would like to add support for proxy protocol (https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt) in actix-web.

Add this support would extend the use of actix to e.g. aws elb, google tcp load balancer.

If someone can point out the code to start with, I can try to do this task as well.