Add nbgrader extension magic

Allows cells with jinja templates with the "solution" variable to be rendered.
> Oops, yeah. I am definitely going to make the switch soon but I am working on a project that still depends on a library that is python 2 only so that's what I'm still used to coding in :-/

I forgive you then ! :-) Are you officially working on nbgrader, or just on spare time ?