config: add create_on_announce option

This option allows the user to specify whether or not to create a new
swarm for torrents that do not previously exist within the tracker's
storage.

tracker/announce.go

@@ -27,7 +27,7 @@ func (tkr *Tracker) HandleAnnounce(ann *models.Announce, w Writer) (err error) {
 
 	torrent, err := tkr.FindTorrent(ann.Infohash)
 
-	if err == models.ErrTorrentDNE && !tkr.Config.PrivateEnabled {
+	if err == models.ErrTorrentDNE && cfg.CreateOnAnnounce {
 		torrent = &models.Torrent{
 			Infohash: ann.Infohash,
 			Seeders:  models.NewPeerMap(true, tkr.Config),