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.
The original [`#initialize`](https://github.com/mbj/anima/blob/master/lib/anima.rb#L110) does NOT provide a default argument. I dislike that an anima infected class with defaults has a more permissive interface than a anima infected class without defaults. This violates the LSP.

There are two options:

* Remove the default argument. This would be kinda ugly in case the default attributes fill all required attributes in already, resulting in `AnimaInfectedClass.new({})`.
* Provide a default argument of `{}` in the base implementation.

I do not like both solutions that much. I need to think about this a bit, comments welcome.