_prev = function prev() {
        clearTimeout(_this7.playing.timeout);
        removeClass(list[index], CLASS_IN);
        index -= 1;
        index = index >= 0 ? index : total - 1;
        addClass(list[index], CLASS_IN);
        _this7.playing.timeout = setTimeout(_prev, options.interval);
      }
Play the images
@param {boolean|FullscreenOptions} [fullscreen=false] - Indicate if request fullscreen or not.
@returns {Viewer} this