message: wheelTo API index default is 0
diff: import { warn } from '../util/debug'
  export function wheelMixin(BScroll) {
  - BScroll.prototype.wheelTo = function (index) {
  + BScroll.prototype.wheelTo = function (index = 0) {
  if (this.options.wheel) {
  this.y = -index * this.itemHeight
  this.scrollTo(0, this.y)
  