Feature request: onBefore

[rottmann]

When click a link and before route to the target, it would be nice to have a global onBefore function or a onBefore on each route (or both).

E.g. if a page has modified fields, you can ask in a modal to save before leaving the page.

Currently you must implement it on your own on each link

<script>
function beforeSwitch() {
  if (isModified && alert('Leave?')) {
    const url = new URL(this.href)
    push(url.hash.substr(1)) // remove # and route to target
  }
}
</script>

<a href="#/target" on:click|preventDefault={beforeSwitch}>Link</a>

[ItalyPaleAle]

This is great timing :) I created #40 as a draft last night and would be great if you could provide feedback on that design.

(Closing as duplicate of #23 and #40)