add aerc: a better terminal mail client

TODO: home-manager integration. its config file doesn't obviously
support passwords, but i could possibly encode it as a secret and
bind-mount the secret location, or symlink it?

modules/universal/home-manager.nix

@@ -259,6 +259,7 @@ in
       with pkgs;
       [
         # GUI only
+        aerc  # email client
         audacity
         chromium
         clinfo