Scrolling behind modal

Hi,

I'm using sweet-scroll (it's smooth as butter, by the way!) inside a tall modal, however, `toElement` scrolls both the modal and the window behind it.

```javascript
// In context of React using hooks
const scroller = new SweetScroll({
  duration: 1000,
  easing: "easeOutExpo"
}, feedRef.current)
scroller.toElement(el.current, {offset: -75})
```

Any suggestions?