syncthing: 0.11 -> 0.12

Also, keep 0.11 around (in an updated version) and make the
pkg an option to the service module.

nixos/modules/services/networking/syncthing.nix

@@ -21,7 +21,7 @@ in
         description = ''
           Whether to enable the Syncthing, self-hosted open-source alternative
           to Dropbox and BittorrentSync. Initial interface will be
-          available on http://127.0.0.1:8080/.
+          available on http://127.0.0.1:8384/.
         '';
       };
 
@@ -40,6 +40,17 @@ in
         '';
       };
 
+      package = mkOption {
+        type = types.package;
+        default = pkgs.syncthing;
+        example = literalExample "pkgs.syncthing";
+        description = ''
+          Syncthing package to use.
+        '';
+      };
+
+
+
     };
 
   };
@@ -66,7 +77,7 @@ in
         };
       };
 
-    environment.systemPackages = [ pkgs.syncthing ];
+    environment.systemPackages = [ cfg.package ];
 
   };