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