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? e.g.

```
> ## What you need to know {.prereq}
>
> not very much

> ## What you will learn {.objectives}
>
> lots of things
```