Propagate events

[Rich-Harris]

The corollary to #4 — there's currently no way to listen for events coming from a row component. One way would be to have a single catch-all event:

<VirtualList on:event='handleEvent(event)' ... />

In that situation, the row component would need to do this sort of thing:

<div on:click="fire('event', { type: 'select', row })">
  <!-- row markup here -->
</div>

It would be slightly nicer if the row component could look like this...

<div on:click="fire('select', { row })">
  <!-- row markup here -->
</div>

...but that would require some new way to propagate all events:

<:Component {component} on:* row={{item.data}} />