Implement interactive command quit

[jfkw]

Interactive command quit is documented in README. However, there is no case handler for quit in pkg/cmd func ProcessCmd().

The interactive tcell loop in cmd/uchess func Interact() handles Ctrl-C, so there is currently a way to quit the app.

When implementing, avoid coupling pkg/cmd and cmd/uchess such that they both handle process quit concerns with overlap. The goal should be for pkg/cmd to handle game state shutdown, and cmd/uchess to handle process shutdown.