Every January 1 throughout the world, hordes of human "polar bears" make a mad dash to icy waters for a quick Plunge.   Why do they brave the elements and the possibility of frostbite and hypothermia?  Some say it’s a way to combat depression.  Others claim it is a spiritual experience, but I suspect it’s even more than that.  They take the plunge for one of the same reasons writers write: they want to prove they’re alive.     From a writing perspective, winter has always been a productive