Implements a basic way to 'group' affixes together.

An affix group is a collection of affixes which generally provide the
same benefits, but at varying levels. For example, '+2 strength' and '+4
strength' would be considered grouped affixes, assuming that they were
spawning on the same items and such

AffixLister got refactored some more, in preparation for adding the
ability to load things that aren't affixes.