Mental Illnesses [WIP]

Adds a basic framework for mental illnesses, so the psychologist
actually has something to do. Mental illnesses are non-contagious,
invisible diseases that crew members have a chance to spawn with. Crew
that start with a mental illness will have an empty bottle of pills in
their backpack.

There's a lot that can still be done, but what it needs most right now is feedback, especially suggestions or criticisms.