The font vars went well, so I also added general vars.

I created the blurb.  I didn't know if it would work with no body, so I just put a placeholder.

I did have another question.  Instead of just one tracers.add() method, I was wondering if there's an advantage to having one called add_callback(var, callback) and a separate one called add_default(var, config)?  Design-wise, I don't know which is the preferred way, but changing the existing code seemed a little hackish.