Controller Support

Added partial controller support! It wasn't really feasible to add controller support to the main menu because of its unique layout... but all aspects of gameplay now have controller support. That means if you have a controller with analog triggers, you can choose how fast you move when accelerating. Same with turning: you can now turn a lot more accurately with a control stick. 

Keyboard and gamepad are always enabled, so feel free to just swap whenever you want. No setting change needed.