I've been thinking about how to have readers "test" their knowledge in more interesting ways as they finish a tutorial. This is a first proof of concept for having a challenge. My goal on this PR is to get comments so we can move forward as we're building content for the next wave.

Things I like:

1. The general structure of the challenge: A reader must write some code that performs some work. Something like a test harness runs that code with a variety of inputs. The reader gets feedback on what's right, and what's not.
2. The tuple for input / expected. I think that can get generalized. Caveat: Not sure of an elegant way to mark inputs that should generate exceptions.
3. Giving hints. This could be generalized to go through successive pages and show more hints if someone is stuck (especially for larger challenges).

Things I don't like:

1. The problem: It's really boring. I wanted to limit the scope to methods and capabilities that readers have seen in this tutorial. That's very limiting in such a beginner topic.
2. ** Almost giving the answer.** Related to the first point. I gave a strong hint because the method hadn't been covered.