[feature request] Support proxy protocol

[thetechnick]

I am experimenting with replacing haproxy with gobetween as TCP/UDP lb in front of a kubernetes cluster.
For most of my deployments the preservation of the client IP is mandatory for logging/auditing and rate limiting on application level.

To preserve the client IP I currently use proxy protocol terminated at the kubernetes ingress controller (nginx).
https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt

Proxy Protocol is implemented by:

• HAProxy
• AWS ELB
• Nginx

So I want to ask if it is possible to include support for proxy protocol in gobetween.
After doing a quick scan through the code base I think it would be very simple to add the header information to the connection handling about here:
https://github.com/thetechnick/gobetween/blob/master/src/server/tcp/server.go#L345

[yyyar]

Done as #101