First implem for adding classes to enclosing elts.

Extra text representing classes is detected and correctly removed.
Adding attributes isn't working yet.
Uses the right method setAttribute.

Works better, and the html elements get their attributes.
Add attributes extracted from an attribute.

Allows to add attributes to element of an attribute which contains
the attribute pattern.
Cleanup code, remove console log debugs.
Add 'data-element-attributes' attr. to 'section'.

By default '{\\\.\s*?([^}]+?)}'.
Add unit-tests for attributes in mardown elements.

Test paragraphs, list items and img attributes.