prevent calling jump() when current jump is not finished

I created this wrapper, but it could be included in the lib:
```js
import jump from 'jump.js'

let scrolling

const end = () => {
  scrolling = false
}

const scroll = (to, options) => {
  if (scrolling) {
    return
  }
  scrolling = true
  setTimeout(end, options.duration || 0)
  jump(to, options)
}

export default scroll
```