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'm just thinking that we should also check for the default keys being a subset of the original keys. I think the whole code would be easier in case we lift the default functionallity to the main `Anima` module. With an interface of:

```
class Foo
  include Anima.new(:foo, :bar).defaults(foo: 1)
end
```

This has the nice benefit of not having to deal with the case of including an `Anima::Default` instance and NOT including an `Anima` instance.

Also we lower the public *constant* interface this way.