Add Anima::Defaults

Add a way to specify defaults for certain attributes.

Probably not up to your standards, but the basic idea is there. Had some issues running mutant, this will need some more coverage.
BTW: Generally probably define a `EMPTY_HASH = {}.freeze` instance somewhere and pass it in as the default argument like `def initialize(attributes = EMPTY_HASH)` this avoids cases where the default unintentionally gets mutated.