Render the item list
@return {Void}
function() {
  if (this.items.length) {
    var f = document.createDocumentFragment();

    if (this.config.pagination) {
      var pages = this.pages.slice(0, this.pageIndex);

      util.each(pages, function(i, items) {
        util.each(items, function(j, item) {
          appendItem(item, f, this.customOption);
        }, this);
      }, this);
    } else {
      util.each(this.items, function(i, item) {
        appendItem(item, f, this.customOption);
      }, this);
    }

    if (f.childElementCount) {
      util.removeClass(this.items[this.navIndex], "active");
      this.navIndex = f.querySelector(".selectr-option").idx;
      util.addClass(this.items[this.navIndex], "active");
    }

    this.tree.appendChild(f);
  }
}