nixos/light: add minBrightness option

Make the minimum brightness level configurable instead of using an arbitrary value of 0.1.

nixos/modules/programs/light.nix

@@ -49,6 +49,15 @@ in
           '';
         };
 
+        minBrightness = lib.mkOption {
+          type = lib.types.numbers.between 0 100;
+          default = 0.1;
+          description = ''
+            The minimum authorized brightness value, e.g. to avoid the
+            display going dark.
+          '';
+        };
+
       };
 
     };
@@ -63,13 +72,14 @@ in
         let
           light = "${pkgs.light}/bin/light";
           step = builtins.toString cfg.brightnessKeys.step;
+          minBrightness = builtins.toString cfg.brightnessKeys.minBrightness;
         in
         [
           {
             keys = [ 224 ];
             events = [ "key" ];
-            # Use minimum brightness 0.1 so the display won't go totally black.
+            # -N is used to ensure that value >= minBrightness
-            command = "${light} -N 0.1 && ${light} -U ${step}";
+            command = "${light} -N ${minBrightness} && ${light} -U ${step}";
           }
           {
             keys = [ 225 ];