Track heavy rain stuff

This tracks both the wandering monsters, shows rain/thunder/lightning in the charpane, including some info about their regeneration.

It also adds a warning when you might encounter a storm cow.  Right now it fires in all zones, but ideally it would know both water level and terrain type of each zone, and only warn you when a storm cow is a possibility.

I refactored wandering-monsters.lua to abstract the process.  The bees stuff should maybe go in a different file now?