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.

It should be `{. xxxx=yyy }`, not `{_ xxxx=yyy }`.
Only test/test-element-attributes-markdown.html were using `{_`,
simply in order to test the ability, for the user, to define a _different_ pattern.

Also add a description about the solarized theme.

use 'url()'-free path when specifying parallax image, refactor so that parallax is applied to background class, remove unused attributes #595