Add pom (Phase of the Moon) game, inspired by the one in BSD-games
Note that this is my first real Rust program, so it definitely needs code review.
Also: The source branch accidentally includes a commit (4b68aa0e) for Minesweeper. I'm not proficient enough in Git to know how to fix that issue. I have ticked the box to allow commits from members with merge access if anyone doesn't like the change; it makes the game look much nicer on my terminal, but your milage may vary.