vdirsyncer: Add option to request vcard 4.0

modules/programs/vdirsyncer/accounts.nix

@@ -118,6 +118,15 @@ in
       '';
     };
 
+    useVcard4 = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Specifies whether vdirsyncer should request vCards in version 4.0.
+        If set to `false` then vdirsyncer will default to version 3.0.
+      '';
+    };
+
     verify = mkOption {
       type = types.nullOr types.path;
       default = null;

modules/programs/vdirsyncer/default.nix

@@ -101,6 +101,8 @@ let
         end_date = "${v.end}"''
     else if (n == "itemTypes") then
       "item_types = ${listString (map wrap v)}"
+    else if (n == "useVcard4") then
+      ''use_vcard_4 = ${v}''
     else if (n == "userName") then
       ''username = "${v}"''
     else if (n == "userNameCommand") then