Some code blocks weren't indented, so they ended up messing up the numbering. Indenting them fixed it.

renamed markdown attributes for clarity and consistency: data-vertical -> data-separator-vertical, data-notes -> data-separator-notes

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

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.