home: mail: add himalaya

This mail client is very KISS. I like it.

home/mail/himalaya.nix

@@ -0,0 +1,23 @@
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.my.home.mail.himalaya;
+in
+{
+  config.programs.himalaya = lib.mkIf cfg.enable {
+    enable = true;
+
+    settings = {
+      notify-cmd =
+        let
+          notify-send = "${pkgs.libnotify}/bin/notify-send";
+        in
+        pkgs.writeScript "mail-notifier" ''
+          SENDER="$1"
+          SUBJECT="$2"
+          ${notify-send} \
+            -c himalaya \
+            -- "$(printf 'Received email from %s\n\n%s' "$SENDER" "$SUBJECT")"
+        '';
+    };
+  };
+}