Update the typography a bit.

Addresses #20 so the environment uses Proxima Nova Semibold (if it’s available, which it is on KA) for blocks in the editor and toolbox. This is more consistent with the Khan Academy visual style.
Now that I think about it, the length is probably never less than 1, so I bet it’s never even reaching the “0” case. I’ll change this so the min is 1.