Car electrical system expansion

There have been some other vehicle enhancements lately by @ianestrachan, so there are bound to be a few conflicts in here. I'm working right now on updating my fork so I can straighten it all out, but so far we have:

* Car batteries (they don't spawn anywhere yet, including in cars)
* Headlights now require power to turn on
* A framework for powering future powered parts, whenever they're added