nixos/avahi: add support for extraConfig

nixos/modules/services/networking/avahi-daemon.nix

@@ -37,6 +37,7 @@ let
 
     [reflector]
     enable-reflector=${yesNo reflector}
+    ${extraConfig}
   '';
 
 in
@@ -176,6 +177,14 @@ in
         '';
       };
 
+      extraConfig = mkOption {
+        default = "";
+        type = types.lines;
+        description = ''
+          Extra config to append to avahi-daemon.conf.
+        '';
+      };
+
     };
 
   };