Add something about resizing/recentering the screen in the Game Of Life problem

[NicMcPhee]

Part 4 ends with a list of ways we could fancy this up. I should add something to that list about an extension that resizes and recenters the "screen" around whatever cells are currently alive. It's possible for the "action" to drift off-screen in a way where most (even all!) of the screen is blank but there are live cells off-screen doing interesting things.

This could get weird if there are gliders, for example, so there would probably need to be some sort of maximum screen size and some notion of where the "center of mass" of the current set of live cells is.