Navigate through the dropdown
@param  {obj} e
@return {void}
function(e) {
  e = e || window.event;

  // Filter out the keys we don"t want
  if (!this.items.length || !this.opened || !util.includes([13, 38, 40], e.which)) {
    this.navigating = false;
    return;
  }

  e.preventDefault();

  if (e.which === 13) {

    if (this.config.taggable && this.input.value.length > 0) {
      return false;
    }

    return this.change(this.navIndex);
  }

  var direction, prevEl = this.items[this.navIndex];

  switch (e.which) {
    case 38:
      direction = 0;
      if (this.navIndex > 0) {
        this.navIndex--;
      }
      break;
    case 40:
      direction = 1;
      if (this.navIndex < this.items.length - 1) {
        this.navIndex++;
      }
  }

  this.navigating = true;


  // Instead of wasting memory holding a copy of this.items
  // with disabled / excluded options omitted, skip them instead
  while (util.hasClass(this.items[this.navIndex], "disabled") || util.hasClass(this.items[this.navIndex], "excluded")) {
    if (direction) {
      this.navIndex++;
    } else {
      this.navIndex--;
    }

    if (this.searching) {
      if (this.navIndex > this.tree.lastElementChild.idx) {
        this.navIndex = this.tree.lastElementChild.idx;
        break;
      } else if (this.navIndex < this.tree.firstElementChild.idx) {
        this.navIndex = this.tree.firstElementChild.idx;
        break;
      }
    }
  }

  // Autoscroll the dropdown during navigation
  var r = util.rect(this.items[this.navIndex]);

  if (!direction) {
    if (this.navIndex === 0) {
      this.tree.scrollTop = 0;
    } else if (r.top - this.optsRect.top < 0) {
      this.tree.scrollTop = this.tree.scrollTop + (r.top - this.optsRect.top);
    }
  } else {
    if (this.navIndex === 0) {
      this.tree.scrollTop = 0;
    } else if ((r.top + r.height) > (this.optsRect.top + this.optsRect.height)) {
      this.tree.scrollTop = this.tree.scrollTop + ((r.top + r.height) - (this.optsRect.top + this.optsRect.height));
    }

    // Load another page if needed
    if (this.navIndex === this.tree.childElementCount - 1 && this.requiresPagination) {
      load.call(this);
    }
  }

  if (prevEl) {
    util.removeClass(prevEl, "active");
  }

  util.addClass(this.items[this.navIndex], "active");
}