Add setting for custom nginx config

In my vhosts, I use an included file for an ACME location block. This PR adds the ability to append arbitrary nginx config to the ansible-installed vhost, which allows me to do what I need to do without needing to maintain my own copy of the entire vhost.

It adds a config var, funkwhale_nginx_additional_config, with an empty default.

defaults/main.yml

@@ -30,6 +30,7 @@ funkwhale_nginx_managed: true
 funkwhale_nginx_tls_termination: true
 funkwhale_nginx_max_body_size: 100M
 funkwhale_nginx_use_compression: true
+funkwhale_nginx_additional_config:
 funkwhale_ssl_cert_path:
 funkwhale_ssl_key_path:
 funkwhale_protocol: https