Allow marking parent container to preload all children

[Philipp Daun]

Allow setting the preload attribute on a parent.

<ul data-swup-preload>
  <li><a href="/lorem/">Lorem</a></li>
  <li><a href="/ipsum/">Ipsum</a></li>
  <li><a href="/dolor/">Dolor</a></li>
</ul>

Probable solution:

- queryAll('[data-swup-preload]')
+ queryAll('a[data-swup-preload], [data-swup-preload] a')

[Ji Yong, Kim]

How about data-swup-preload="children"?
Use exact same attribute for different behavior will cause mis-direction.