Add reverse proxy deploy support, fixed issue #59

[MegatronKing]

This commit provides two solutions. Config the nginx like this:

 location / {
    proxy_http_version 1.1;
    proxy_set_header proxy-origin https://pub.example.com/;
    proxy_pass http://localhost:4000/;
}

Another way, we can assign a proxy when starts the server:

unpub --proxy-origin https://pub.example.com