Sandbox mode, alternative color schemes

SIC-1 has been updated in response to player feedback. New features:

• Sandbox Mode (an open-ended task with customizable input, for experimentation/exploration)
• Alternative color schemes (black-on-white, white-on-gray, amber phosphor, and a more accurate P1 green phosphor)
• In-manual ASCII table
• More in-game help near the beginning of the game
• Arrow key navigation for menus
• Many miscellaneous bug fixes