Allow a holistic view of the tutorial code (for Parts II and III)

Some people have pointed out that Part II and Part III can be confusing with their “piecemeal” approach (sometimes people get confused as to what their code is *supposed* to look like after a given lesson, mostly). Having some way to show what the repo should look like at a given point in time would, I believe, solve issues like #110 too?

To do:

- [ ] Figure out UX
- [ ] Implement it