Make auto-hiding solutions machinery for exercises

[eteq]

Right now there's not really a way to include the solution to exercises inside the notebook. What would be ideal would be a way to "hide" cells, so that when someone opens up the notebook, they can hack around with it until they're done, and then click a "show solution" button.

There's some notebook add-ons that do this, but I'm not sure anyone fits this exact need. So it might need to be a custom js addon we need to embed in the notebooks...