feat: allow overriding default 404 response with custom file

This allows folks the ability to restore the previous routing functionality used by static sites backed by VueJS (as an example).

Note that all requests routed this way will respond with a '200 OK' on the server, potentially causing SEO issues.

Closes #53

conf/app-nginx.conf.sigil

@@ -23,7 +23,11 @@ http {
     port_in_redirect off;
 
     location / {
-      try_files $uri $uri/ =404;
+      {{ if ne $.NGINX_DEFAULT_REQUEST "" }}
+        try_files $uri $uri/ /{{ $.NGINX_DEFAULT_REQUEST }};
+      {{ else }}
+        try_files $uri $uri/ =404;
+      {{ end }}
     }
   }
 }