Accept root nodes

[derhuerst]

This PR just uses a <div> element if there are more than 1 children in the root root node.

[wooorm]

Thanks! I amended it a bit, notably:

• not pulling in unist-builder to create a simple JSON structure (div)
• making sure that one child is an element (comments, doctypes, texts would cause errors)
• allowing a root without children, instead of throwing
• adding docs
• changed the tests a bit to maintain style

But awesome, thanks for working on it!