gallery-dl: add module

gallery-dl [1] is a cli to download image galleries from several image
hosting sites.

[1] https://github.com/mikf/gallery-dl

modules/programs/gallery-dl.nix

@@ -0,0 +1,41 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.programs.gallery-dl;
+
+  jsonFormat = pkgs.formats.json { };
+
+in {
+  meta.maintainers = [ maintainers.marsam ];
+
+  options.programs.gallery-dl = {
+    enable = mkEnableOption "gallery-dl";
+
+    settings = mkOption {
+      type = jsonFormat.type;
+      default = { };
+      example = literalExpression ''
+        {
+          extractor.base-directory = "~/Downloads";
+        }
+      '';
+      description = ''
+        Configuration written to
+        <filename>$XDG_CONFIG_HOME/gallery-dl/config.json</filename>. See
+        <link xlink:href="https://github.com/mikf/gallery-dl#configuration"/>
+        for supported values.
+      '';
+    };
+  };
+
+  config = mkIf cfg.enable {
+    home.packages = [ pkgs.gallery-dl ];
+
+    xdg.configFile."gallery-dl/config.json" = mkIf (cfg.settings != { }) {
+      source = jsonFormat.generate "gallery-dl-settings" cfg.settings;
+    };
+  };
+}