Code Cleanup, More Options and a Bugfix.

[Ceikry]

Changes Proposed:

• Refactor of InvType and Subclass mismatch handling to allow for easier future extensibility
• (Transmogrification.AllowLowerTiers) Add the ability to transmog into any tiered armor class the user can actually equip (e.g. Plate users can use all tiers, so can transmog into items from any tier, leather users can only use leather and cloth so can tmog into leather and cloth, etc.)
• (Transmogrification.AllowMixedOffhandArmorTypes) Add the ability to transmog offhand armor types into any other offhand armor type (e.g. "Held in Off-hand" gear over shields and vice versa)
• Allow MISC-subclass armor to be transmogrified over any armor tier as long as their types match (Helm and Helm, etc)

Issues Addressed:

• Fixes Cosmetic armour pieces without type arent transmoggable. #147

Tests Performed:

• Tested default config - works as expected
• Tested AllowLowerTiers by itself - works as expected
• Tested AllowMixedOffhandArmorTypes by itself - works as expected
• Tested AllowLowerTiers, AllowMixedOffhandArmorTypes together - works as expected
• Tested AllowMixedArmorTypes by itself - works as expected
• Tested AllowMixedWeaponTypes 0,1,2 by itself - works as expected
• Tested transmogging cosmetic items like the Forever Lovely Rose over any other headpiece - works as expected

How to Test the Changes:

• Compile the module as normal
• Test default transmog rules - only certain tiers allowed (source and target) by default, can only tmog Mail into Mail, Plate into Plate, etc.
• Test AllowLowerTiers 1 - Should allow Plate wearers to tmog into Plate, Mail, Leather, Cloth. Mail into (Mail|Leather|Cloth). Leather into (Leather|Cloth) and Cloth into Cloth.
• Test AllowMixedOffhandArmorTypes 1 - Should allow transmogging a shield over a "Held in Off-hand" item and vice versa
• Test cosmetic pieces like those mentioned in the linked issue - should be able to transmog them over standard, non-cosmetic items.

Sorry, I know multiple things in one PR is bad practice but all 3 things just happened to be related to the same region of code I was working on.