function getOppositePlacement(placement) {
    var hash = { left: 'right', right: 'left', bottom: 'top', top: 'bottom' };
    return placement.replace(/left|right|bottom|top/g, function (matched) {
      return hash[matched];
    });
  }
Get the opposite placement of the given one
@method
@memberof Popper.Utils
@argument {String} placement
@returns {String} flipped placement