Fix slim/html5/section – use :sectnumlevels attribute

It’s just copied it from [HAML template](https://github.com/asciidoctor/asciidoctor-backends/blob/master/haml/html5/section.html.haml#L16) for now, I’ll rewrite it using helper method after merging [asciidoctor#1143](https://github.com/asciidoctor/asciidoctor/pull/1143).