Now the transition isn’t any more „per slide“ but „per movement of a
slide“. So the slide can disappear in another way than it appeared.

To honor the DRY principle repeating patterns have been factored out.
In addition there was an anchor missing in README.md.

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.