add offset prop to Waypoint component

[milocosmopolitan]

Hello @matyunya, thanks for this library. It really help my site performance so much with image lazy loading with compressed images.

However, I've ran into a scenario of using the svelte-image component with some animations to be triggered on certain scollY value. I see you already have this offset prop in svelte-waypoint.

This will help me to load the image that is below the specific offset height, so that when animation is starting in the viewport image is already loaded.

[matyunya]

Thanks!