- Added a new "Formula" attribute type for both actors and items.
- When iterating through attributes, formula attributes are noted in an
array so that they can be iterated through in a second pass to
evaluate their referenced attributes. The second iteration just
handles attribute replacement rather than rolling the formula, that's
deferred until the attributes are used in an actual roll formula.
- Items also have the same behavior. If an attribute isn't on the item,
it will fall back to check the parent actor instead.
- Likely areas with issues would be formula attributes that reference
other formula attributes, or item formula attributes that reference
actor formula attributes.
