settings: make characters used in visualizer customizable

src/settings.h

@@ -174,6 +174,7 @@ struct NcmpcppConfig
 	std::string new_header_second_line;
 	std::string lastfm_preferred_language;
 	std::basic_string<my_char_t> progressbar;
+	std::basic_string<my_char_t> visualizer_chars;
 	
 	std::string pattern;