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.
I think for such a simple initializer you should use [concord](https://github.com/mbj/concord). This has the nice benefit of having instances of `Anima::Defaults` comparable to each other via equalizer that concord includes transitively.