Consider adding `rootMargin` to the Intersection Observer

Feedback from @surma:

> I feel like by default you should add a rootMargin to the IO to trigger the load before something comes into the viewport.

So, `rootMargin` defaults to "0px 0px 0px 0px", meaning the intersection will be computed between the root element's unmodified bounds rectangle and the target's bounds. This change would be made to https://github.com/GoogleChromeLabs/quicklink/blob/master/src/index.mjs#L21