Add initial support for loading JokerPP savegames

See #123

src/saveload/extended_ver_sl.h

@@ -84,6 +84,7 @@ enum SlXvFeatureIndex {
 	XSLFI_TRAFFIC_LIGHTS,                         ///< This save game uses road traffic lights
 	XSLFI_RAIL_AGEING,                            ///< This save game uses the rail aging patch
 	XSLFI_SPRINGPP,                               ///< This is a SpringPP game, use this for loading some settings
+	XSLFI_JOKERPP,                                ///< This is a JokerPP game, use this for loading some settings
 
 	XSLFI_SIZE,                                   ///< Total count of features, including null feature
 };