open(index) {
      let $collapsibleLi = this.$el.children('li').eq(index);
      if ($collapsibleLi.length && !$collapsibleLi[0].classList.contains('active')) {
        // onOpenStart callback
        if (typeof this.options.onOpenStart === 'function') {
          this.options.onOpenStart.call(this, $collapsibleLi[0]);
        }

        // Handle accordion behavior
        if (this.options.accordion) {
          let $collapsibleLis = this.$el.children('li');
          let $activeLis = this.$el.children('li.active');
          $activeLis.each((el) => {
            let index = $collapsibleLis.index($(el));
            this.close(index);
          });
        }

        // Animate in
        $collapsibleLi[0].classList.add('active');
        this._animateIn(index);
      }
    }
Open Collapsible
@param {Number} index - 0th index of slide