ci status indicator is svg instead of png

fix small misspelling: Mardown -> Markdown

tweak layout in readme installation markup

Some code blocks weren't indented, so they ended up messing up the numbering. Indenting them fixed it.

For the Moon and Solarized themes they had .css appended to them which was causing them to not come up.

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

- 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'

- 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.