students modify existing code instead of ext files

Updates to lesson 1. Previously students would edit an external
file that would be open()'d, read, and exec()'d by the main app.
This would be confusing to python newcomers if they ever dug into
how the lessons worked.

Instead, the new approach will guide students to make a small
adjustment to an existing python game. This should be more clear
and ideally even stoke curiousity about how the game itself works.