Consider supporting "ref" prop

Consider ref props on marks for getting access to low-level renderables:

```
const ref = useRef<SVGRectElement>(null)

<Rect ref={ref} />
```

On bound marks, I'm not sure what this would look like - maybe an array of items?
```
const ref = useRef<SVGRectElement[]>([])
<Rect ref={ref} />
```