[Bug Report] [Bug Report]countup组件,监听start属性也没法重复激发效果

### VUX version
2.9.2

### OS/Browsers version
macOS

### Vue version
2.2.2

### Code

```html
options: {
navigation: true,
loopBottom: true,
afterLoad: function(from, to, direction) {
if (to.index === 1) {
this.pageTwo.doStart = true;
} else if (to.index === 2) {
this.pageThree.doStart = true;
} else if (to.index === 4) {
this.pageFive.doStart = true;
} else if (to.index === 5) {
this.pageSix.doStart = true;
}
}.bind(this),
onLeave: function(from, to, direction) {
if (from.index === 1) {
this.pageTwo.doStart = false;
} else if (from.index === 2) {
this.pageThree.doStart = false;
} else if (from.index === 4) {
this.pageFive.doStart = false;
} else if (from.index === 5) {
this.pageSix.doStart = false;
}
}.bind(this)
},
```

### Steps to reproduce
在本人一个fullpage组件内,进入或者离开页面会分别触发事件，在所触发的事件中改变vux countUp组件的start值,并没有重新激发countUp组件的效果，只在第一次进入时有效。
在监听start值时，调用一次countupjs的reset()方法，能解决问题。
watch: {
start (val) {
if (val) {
this._countup.reset()
this._countup.start()
}
},
}

### What is Expected?
监听start属性能够重复激发效果

### What is actually happening?
监听start属性没法重复激发效果