Allow users to add custom http-level nginx config

This is particularly useful if you need to add realip config, to ensure
Privatebin's IP-based rate limiting works correctly.

files/nginx.conf

@@ -47,6 +47,8 @@ http {
         font/opentype
         image/svg+xml;
 
+    include /etc/nginx/conf.d/*.conf;
+
     server {
         listen 80;
         root /var/www;