Add support for custom NGINX config

Including *.conf files in /etc/nginx/conf.d same as the default NGINX
configuration gives the user more flexibility.

core/nginx/conf/nginx.conf

@@ -248,6 +248,8 @@ http {
         proxy_pass http://$admin/internal$request_uri;
       }
     }
+
+    include /etc/nginx/conf.d/*.conf;
 }
 
 mail {