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.
There is already a getter for the anima module. `AnimaInfectedClass.anima` returning an `Anima` instance.

This code adds an other getter `AnimaInfectedClass.anima_defaults`, this is kind of aname duplication with the `.anima` method above, we could avoid in case we refactor the code to attach itself to the `Anima` module instance. Defaults would be accessed via `AnimaInfectedClass.anima.defaults`. Resulting in minimum namespace cluttering.