fixed two minor bugs, a twohanded gesture may not have a direction, and if autoCenter is of, the tipPosition was in the wrong scope

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

- 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.