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

- 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'

It is important that test/test-element-attributes-markdown.html do test
the ability for the user to define its own pattern.

Test paragraphs, list items and img attributes.