* Print background in pdf

Pull request to integrate issue https://github.com/hakimel/reveal.js/issues/1686. Adding the workaround to the theme template to make it work for every theme.

* apply suggestion of @hakimel

This reverts commit b7e0d9b1.

`<tbody/>` is automatically inserted, so we can guarantee that this affects all ths and tds in the last row of the table *body* and the head is left untouched

The alignment information were already in the generated html.
The css was the only thing missing, so I added it.

css/theme/template/settings.scss adds the $headingMargin variable, set to `0 0 20px 0`.
css/theme/template/theme.scss uses $headingMargin to set margin on headers.