For coherence I renamed the markdown filter file in order to add some
other converters if necessary. I added a htmlattrs filter file that can
surcharge any type of html tag that would be generated by Django and
avoid to modify the code in a view or a form with css or other needs.
Every modification takes place in the template.

Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>