Unified Attribute syntax for slides and elements generated from markdown

After #677 and #640:

- I simplified the implementation of "adding attributes on elements or slides generated from markdown".  
- I used an unified syntax

````
<!-- .slide: xx=yy ww=zz -->
<!-- .element: xx=yy ww=zz -->
````

See:

- https://github.com/VonC/reveal.js/blob/attributes/README.md#element-attributes
- https://github.com/VonC/reveal.js/compare/dev...attributes to see the difference between the previous and the current implementation