function lastChild(el, selector) {
    var last = el.lastElementChild;
    while (last && (last === Sortable.ghost || css(last, 'display') === 'none' || selector && !matches(last, selector))) {
      last = last.previousElementSibling;
    }
    return last || null;
  }
Gets the last child in the el, ignoring ghostEl or invisible elements (clones)
@param  {HTMLElement} el       Parent element
@param  {selector} selector    Any other elements that should be ignored
@return {HTMLElement}          The last child, ignoring ghostEl