function clockwise(placement) {
    var counter = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : false;

    var index = validPlacements.indexOf(placement);
    var arr = validPlacements.slice(index + 1).concat(validPlacements.slice(0, index));
    return counter ? arr.reverse() : arr;
  }
Given an initial placement, returns all the subsequent placements
clockwise (or counter-clockwise).

@method
@memberof Popper.Utils
@argument {String} placement - A valid placement (it accepts variations)
@argument {Boolean} counter - Set to true to walk the placements counterclockwise
@returns {Array} placements including their variations