Activate the following cheats by entering the corresponding codes at the title 
screen. Note: These work on both PC and Mac:

CHEAT FUNCTION                 CHEAT CODE
-----------------------------------------
invincibility                - UP, DOWN, UP, DOWN, LEFT(2), RIGHT(2)
plays a secret chiptune      - LEFT, RIGHT, LEFT, RIGHT, LEFT, RIGHT, LEFT, RIGHT, 
shows FPS rate on the screen - DOWN(4), LEFT(4), RIGHT(4)
unlocks all the levels       - UP(3), DOWN(3), LEFT, RIGHT