ItemEnchantabilityEvent and ItemRarityEvent

2 new events allowing modification of ItemStacks' rarity and enchantability values.  Each event provides the ItemStack in question, as well as the default value.

Enchantability is used during enchanting and repairing, while Rarity is (within vanilla mechanics) primarily a cosmetic value, affecting the color of the item's name in the tooltip, but could also be expanded upon for new, interesting mechanics.