Proxy protocol support.

Hello :) I have GitLab running behind HAproxy. It's using the proxy protocol (http://www.haproxy.org/download/1.5/doc/proxy-protocol.txt), which nginx supports. The main thing needed to enable this is for nginx to have proxy_protocol listed on the listen directive.

Example: listen *:80 proxy_protocol;

Unfortunately, I don't see a clean way to add that from within gitlab.rb. Can there be a configuration parameter for this? It could just be a boolean or a custom extension.