Only fix Headroom once user has scrolled beyond threshold

[madeleineostoja]

Hey thanks for the great component. It would be great if svelte-headroom began statically positioned, and created a spacer element so that the header could become fixed positioned once the user scrolls past the triggering threshold without reflowing the page. This is similar to how react-headroom works.

Currently svelte-headroom is always fixed position, leaving making an offset for the header up to the user which is less than ideal.