Allow mods, trailers, and boats to be spawned in with vehicles

[DeckardCain001]

Enables several new config options for the vehicle spawn menus. When spawning in a vehicle, you can now configure mods for that vehicle, a trailer (with options, automatically attached to vehicle), and a boat (with options, automatically attached to the trailer). The configurable 'boat' option can be used to attach other types of vehicles to a trailer as well, such as an ATV.