Keepalive agents

[jjfiv]

• [ ] breakout: always hit the ball
• [ ] amidar: don't hit enemies
• [ ] gridworld: random steps, don't step on death states
• [ ] space_invaders: always dodge lasers, fire at random

[jjfiv]

Vision: Add a field "keep_alive" to the game state. On update, if no buttons are pressed and keep_alive, choose a button press semi-intelligently.

Good-first-issue in terms of focusing on one game.

[etosch]

Handled in a separate repository.