Added Progressive Mode!

Big update today!

Added "Progressive Mode", a longer mode that has blocks descending towards the bottom of the screen, slowly at first, but accelerating over time. If a block goes off the screen, it's game over!

Hold down the fire button for 5 seconds to return to the main menu and select a different mode.

Please let me know what you think of Progressive Mode. The visuals and UI are still very rough, but I figured getting feedback on the gameplay now would be great.