- new default separator for slide attributes
- new default separator for element attributes
Illustrate both, in html comments.

- Reformat
- Use new default separator format: '.element: x=y'
- Add new tests for testing that default format

- Reformat.
- Change default separator test 'slide: x=y'

- on slides: .slide x=y
- on elements: .element x=y

Those are detected in html comment, in order to not be visible
if a markdown is edited in an html editor.

according to README, I don't include reveal.min.js.
Hence I believe it makes sense to not include reveal.min.css either.
Hence, examples added in index.html can give the feeling that they don't work yet.