yabar: add module

To make the configuration of `yabar` more pleasant and easier to
validate, a NixOS module will be quite helpful.

An example config could look like this:

```
{
  programs.yabar = {
    enable = true;
    bars.top.indicators.exec = "YA_DATE";
  };
}
```

The module adds a user-controlled systemd service which runs `yabar` after
starting up X.

nixos/modules/programs/yabar.nix

@@ -0,0 +1,149 @@
+{ lib, pkgs, config, ... }:
+
+with lib;
+
+let
+  cfg = config.programs.yabar;
+
+  mapExtra = v: lib.concatStringsSep "\n" (mapAttrsToList (
+    key: val: "${key} = ${if (isString val) then "\"${val}\"" else "${builtins.toString val}"};"
+  ) v);
+
+  listKeys = r: concatStringsSep "," (map (n: "\"${n}\"") (attrNames r));
+
+  configFile = let
+    bars = mapAttrsToList (
+      name: cfg: ''
+        ${name}: {
+          font: "${cfg.font}";
+          position: "${cfg.position}";
+
+          ${mapExtra cfg.extra}
+
+          block-list: [${listKeys cfg.indicators}]
+
+          ${concatStringsSep "\n" (mapAttrsToList (
+            name: cfg: ''
+              ${name}: {
+                exec: "${cfg.exec}";
+                align: "${cfg.align}";
+                ${mapExtra cfg.extra}
+              };
+            ''
+          ) cfg.indicators)}
+        };
+      ''
+    ) cfg.bars;
+  in pkgs.writeText "yabar.conf" ''
+    bar-list = [${listKeys cfg.bars}];
+    ${concatStringsSep "\n" bars}
+  '';
+in
+  {
+    options.programs.yabar = {
+      enable = mkEnableOption "yabar";
+
+      package = mkOption {
+        default = pkgs.yabar;
+        example = literalExample "pkgs.yabar-unstable";
+        type = types.package;
+
+        description = ''
+          The package which contains the `yabar` binary.
+
+          Nixpkgs provides the `yabar` and `yabar-unstable`
+          derivations since 18.03, so it's possible to choose.
+        '';
+      };
+
+      bars = mkOption {
+        default = {};
+        type = types.attrsOf(types.submodule {
+          options = {
+            font = mkOption {
+              default = "sans bold 9";
+              example = "Droid Sans, FontAwesome Bold 9";
+              type = types.string;
+
+              description = ''
+                The font that will be used to draw the status bar.
+              '';
+            };
+
+            position = mkOption {
+              default = "top";
+              example = "bottom";
+              type = types.enum [ "top" "bottom" ];
+
+              description = ''
+                The position where the bar will be rendered.
+              '';
+            };
+
+            extra = mkOption {
+              default = {};
+              type = types.attrsOf types.string;
+
+              description = ''
+                An attribute set which contains further attributes of a bar.
+              '';
+            };
+
+            indicators = mkOption {
+              default = {};
+              type = types.attrsOf(types.submodule {
+                options.exec = mkOption {
+                  example = "YABAR_DATE";
+                  type = types.string;
+                  description = ''
+                     The type of the indicator to be executed.
+                  '';
+                };
+
+                options.align = mkOption {
+                  default = "left";
+                  example = "right";
+                  type = types.enum [ "left" "center" "right" ];
+
+                  description = ''
+                    Whether to align the indicator at the left or right of the bar.
+                  '';
+                };
+
+                options.extra = mkOption {
+                  default = {};
+                  type = types.attrsOf (types.either types.string types.int);
+
+                  description = ''
+                    An attribute set which contains further attributes of a indicator.
+                  '';
+                };
+              });
+
+              description = ''
+                Indicators that should be rendered by yabar.
+              '';
+            };
+          };
+        });
+
+        description = ''
+          List of bars that should be rendered by yabar.
+        '';
+      };
+    };
+
+    config = mkIf cfg.enable {
+      systemd.user.services.yabar = {
+        description = "yabar service";
+        wantedBy = [ "graphical-session.target" ];
+        partOf = [ "graphical-session.target" ];
+
+        script = ''
+          ${cfg.package}/bin/yabar -c ${configFile}
+        '';
+
+        serviceConfig.Restart = "always";
+      };
+    };
+  }