* Allow including custom nginx conf files
Give advanced users more flexibility by allowing them to include custom config files at differents locations in the nginx configuration.
`/data/nginx/custom/root.conf`: Included at the very end of nginx.conf
`/data/nginx/custom/http.conf`: Included at the end of the main `http` block
`/data/nginx/custom/server_proxy.conf`: Included at the end of every proxy `server` block
`/data/nginx/custom/server_redirect.conf`: Included at the end of every redirection `server` block
`/data/nginx/custom/server_stream.conf`: Included at the end of every stream `server` block
`/data/nginx/custom/server_stream_tcp.conf`: Included at the end of every TCP stream `server` block
`/data/nginx/custom/server_stream_udp.conf`: Included at the end of every UDP stream `server` block
* Don't fail if file doesn't exist
* Advanced Nginx settings doc
