Vehicles can come with items!

This PR allows vehicles to have items spawn inside of parts with the CARGO flag, with a certain chance. As a proof-of-concept, the driver's seat of a car has a 16% chance of having a pack of cigarettes (since a quick google says that 16.1% of adults in Massachusetts are smokers).

I'm now putting this up so that everyone can throw requests at me for other items to spawn in vehicles - just comment on this PR with a suggested combination of x+y location, item, and % chance of spawning.