Add joystick support

First two game controllers control the players in addition to the
keyboard. Analog axis to move, any button to grab.

Potential problems:
- No dead zone. Controllers with jitter might show phantom movement.
- No handling for more than 2 controllers; may crash w/bad array index.

Pretty crude code hacked together in under an hour by a non-Lua programmer. :facepunch: :hamster: