Simpler default syntax for markdown attributes.

- 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.
test-markdown-attributes.*, new default separator

- Reformat.
- Change default separator test 'slide: x=y'
test-markdown-element-attributes: formats and test.

- Reformat
- Use new default separator format: '.element: x=y'
- Add new tests for testing that default format
README.md mentions the new default separators.

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