Add Transmission BitTorrent service

Enable it with

  services.transmission.enable = true;

and optionally configure it

  services.transmission.settings =
    {
      download-dir = "/srv/torrents/";
      incomplete-dir = "/srv/torrents/.incomplete/";
      incomplete-dir-enabled = true;
      rpc-whitelist = "127.0.0.1,192.168.*.*";
      # for users in group "transmission" to have access to torrents
      umask = 2;
    };

The above settings are written/merged into settings.json each time the
service is about to start.