Is there any case where `getBoundingClientRect` isn't available yet Dragdealer continues to function? If I understand correctly, that would always return [0, 0] which wouldn't make sense for the following piece of code

```js
    this.offset.mouse = [
      Cursor.x - Position.get(this.handle)[0],
      Cursor.y - Position.get(this.handle)[1]
    ];
```
If the entire lib doesn't work without `getBoundingClientRect`, which seems [reasonable](https://developer.mozilla.org/en-US/docs/Web/API/element.getBoundingClientRect#Browser_compatibility), making it optional is pretty misleading. Let me know if I understood correctly.