Create a hook and component for subscribing to updates

[RubenVerborgh]

We should create a hook that listens to NSS Web Sockets (https://github.com/solid/solid-spec/blob/master/api-websockets.md) and returns the last update.

For instance:

const { document, date } = useLastUpdate('http://example.org/foo', 'http://example.org/bar');

and, with a small component wrapper:

<TrackUpdates subscribe="http://example.org/foo,http://example.org/bar">
  <MyContent />
</TrackUpdates>