Do not register SoundCloud/YouTube parsers if no auth details given for them.

main.go

@@ -74,12 +74,17 @@ func main() {
 	m := manager.NewManager()
 
 	// Load youtube parser
+	if len(youtubeApiKey) > 0 {
 		youtubeParser := &youtube.Parser{
 			Config: &youtube.Config{ApiKey: youtubeApiKey},
 		}
 		must(m.RegisterParser(youtubeParser))
+	} else {
+		log.Println("No YouTube API key provided, YouTube parsing via API is disabled.")
+	}
 
 	// Load soundcloud parser
+	if len(soundcloudClientId) > 0 && len(soundcloudClientSecret) > 0 {
 		soundcloudParser := &soundcloud.Parser{
 			Config: &soundcloud.Config{
 				ClientId:     soundcloudClientId,
@@ -87,6 +92,9 @@ func main() {
 			},
 		}
 		must(m.RegisterParser(soundcloudParser))
+	} else {
+		log.Println("No SoundCloud client ID or secret provided, SoundCloud parsing via API is disabled.")
+	}
 
 	// Load wikipedia parser
 	must(m.RegisterParser(new(wikipedia.Parser)))