code: function next() {
      var loop = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : false;
  
      var maxIndex = this.length - 1;
      var index = this.index + 1;
  
      if (index > maxIndex) {
        index = loop ? 0 : maxIndex;
      }
  
      this.view(index);
      return this;
    }
docs: View the next image
  @param {boolean} [loop=false] - Indicate if view the first one
  when it is the last one at present.
  @returns {Viewer} this