add --config flag to modify config path

+ use getters instead of constants in settings classes
+ refactor previous references
+ move get_xdg_config_dir() from settings.py to app.utils.xdg_utils.py

app/print_help.py

@@ -9,7 +9,8 @@ Options:
     {args.build}: Build the application (in development)
     {args.host}: Set the host
     {args.port}: Set the port
-
+    {args.config}: Set the config path
+    
     {', '.join(args.show_feat)}: Do not extract featured artists from the song title
     {', '.join(args.show_prod)}: Do not hide producers in the song title