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 "transmission" group to have access to the torrents:
      umask = 2
    };