Don't render tags as 'self-closing' when not allowed.

Only some tags, like 'br' and 'img', can be "self-closing" in HTML5.
These are not actually self-closing: `<br />` means the same as
`<br>`. But e.g. `<a></a>` means something completely different from
`<a>`, so it should not render this way, even if it has no children.

This fixes #29.