allow createElement to return DocumentFragment?

`nanohtml` now supports document fragments, which are very handy for returning a collection of siblings without having to wrap them in an arbitrary element.

However `nanocomponent` is not currently designed to handle this.

If we modify the `el instanceof window.Element` assert in `_handleRender` to also accept `window.DocumentFragment`, we encounter an error because nanocomponent is trying to brand the returned node (which isn't a node).

This also complicates proxying and many other things. I'm not sure how much of a rewrite this would require, but I do think it would be nice to support this feature.

Thoughts?