[FR] Support problem sheets with solutions

[dlfivefifty]

It would be nice to be able to generate notebooks for problem sheets where the solutions can be omitted. I'm imagining something like this:

** Problem 5** finish the following function that returns the exponential:

function foo(x)
#### SOLUTION
exp(x)
#### ENDSOLUTION
end

Then if I weave in "nosolutions" mode it omits the exp(x) but in "solutions" it includes it.