Car Modifications

[joevenzon]

I can help out with the game/physics code that supports this. I'm not so great with designing the GUI, though -- maybe thelusiv could help with that part. I think the steps to go through are 1) figure out the goals in a fair amount of detail, like which types of parts can be tweaked/upgraded, 2) sketch out the interface, 3) implement the interface, 4) implement the game/physics code.