Small improve at tools/blockquote2div.py

Related to swcarpentry/lesson-template#37.

After you merge this you can merge your `prereq-divs` branch at swcarpentry/lesson-template.
> I think we should preserve all of the classes as special - what if you want a
> learning objective styled div, but with a different title?

I'm OK with support different titles. Do you want that I make this change in
this pull request or do you want to do your self the change?