Rewrite demo assignment

This is a proposed implementation of the assignment layout discussed in #67 .

Resolves #67 
In that case, we could just move the answers to a separate field as well, and just use the `$ ` prompt for tests, since that's how the TestCases are represented anyway. The main reason I had it with comments was because that's probably more intuitive for students and staff. Having separate fields would certainly make it easier for ok.py, though.