message: add autoPullDownRefresh api
diff: @@ -37,4 +37,17 @@ export function pullDownMixin(BScroll) {
  BScroll.prototype.closePullDown = function () {
  this.options.pullDownRefresh = false
  }
  +
  + BScroll.prototype.autoPullDownRefresh = function () {
  + const {threshold = 90, stop = 40} = this.options.pullDownRefresh
  +
  + this.scrollTo(this.x, threshold)
  +
  + if (!this.pulling) {
  + this.pulling = true
  + this.trigger('pullingDown')
  + }
  +
  + this.scrollTo(this.x, stop, this.options.bounceTime, ease.bounce)
  + }
  }
  