[smarkdown] on h1,h2, allow /blabla (in complement to *blabla) to add a container-class

the '*' was problematic: if we wanted to have two of them, it would be interpreted as emphasized text
[smarkdown] do not add classes on h1,h2 if they are set as container-class
[smarkdown] remove attributes on h1,h2 as they get propagated to the containing section