make blocking search constraints change after successful searching optional

it's still enabled by default though.

src/settings.h

@@ -159,6 +159,7 @@ struct ncmpcpp_config
 	bool clock_display_seconds;
 	bool ignore_leading_the;
 	bool stop_after_current_song;
+	bool block_search_constraints_change;
 	
 	int mpd_port;
 	int mpd_connection_timeout;