function _getInsertDirection(target) {
    if (index(dragEl) < index(target)) {
      return 1;
    } else {
      return -1;
    }
  }
Gets the direction dragEl must be swapped relative to target in order to make it
seem that dragEl has been "inserted" into that element's position
@param  {HTMLElement} target       The target whose position dragEl is being inserted at
@return {Number}                   Direction dragEl must be swapped