lib: options: add mkDisableOption

lib/options.nix

@@ -0,0 +1,10 @@
+{ lib, ... }:
+let
+  inherit (lib) mkEnableOption;
+in
+{
+  # Create an option which is enabled by default, in contrast to
+  # `mkEnableOption` ones which are disabled by default.
+  mkDisableOption =
+    description: (mkEnableOption description) // { default = true; };
+}