nixos: add services.bind.extraConfig option

This option allows users to add arbitrary configuration statements into
the generated named.conf file.

nixos/modules/services/networking/bind.nix

@@ -24,6 +24,8 @@ let
         pid-file "/var/run/named/named.pid";
       };
 
+      ${cfg.extraConfig}
+
       ${ concatMapStrings
           ({ name, file, master ? true, slaves ? [], masters ? [] }:
             ''
@@ -110,6 +112,13 @@ in
         }];
       };
 
+      extraConfig = mkOption {
+        default = "";
+        description = "
+          Extra lines to be added verbatim to the generated named configuration file.
+        ";
+      };
+
       configFile = mkOption {
         default = confFile;
         description = "