bottom: add module

Bottom is a cross-platform graphical process/system monitor with a
customizable interface and a multitude of features.

Two unit tests were added validate the module behavior for an empty
configuration and the example configuration.

modules/programs/bottom.nix

@@ -0,0 +1,66 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.programs.bottom;
+
+  tomlFormat = pkgs.formats.toml { };
+
+  configDir = if pkgs.stdenv.isDarwin then
+    "Library/Application Support"
+  else
+    config.xdg.configHome;
+
+in {
+  options = {
+    programs.bottom = {
+      enable = mkEnableOption ''
+        bottom, a cross-platform graphical process/system monitor with a
+        customizable interface'';
+
+      package = mkOption {
+        type = types.package;
+        default = pkgs.bottom;
+        defaultText = literalExample "pkgs.bottom";
+        description = "Package providing <command>bottom</command>.";
+      };
+
+      settings = mkOption {
+        type = tomlFormat.type;
+        default = { };
+        description = ''
+          Configuration written to
+          <filename>$XDG_CONFIG_HOME/bottom/bottom.toml</filename> on Linux or
+          <filename>$HOME/Library/Application Support/bottom/bottom.toml</filename> on Darwin.
+          </para><para>
+          See <link xlink:href="https://github.com/ClementTsang/bottom/blob/master/sample_configs/default_config.toml"/>
+          for the default configuration.
+        '';
+        example = literalExample ''
+          {
+            flags = {
+              avg_cpu = true;
+              temperature_type = "c";
+            };
+
+            colors = {
+              low_battery_color = "red";
+            };
+          }
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    home.packages = [ cfg.package ];
+
+    home.file."${configDir}/bottom/bottom.toml" = mkIf (cfg.settings != { }) {
+      source = tomlFormat.generate "bottom.toml" cfg.settings;
+    };
+  };
+
+  meta.maintainers = [ maintainers.polykernel ];
+}