* Updated `fica.Key` to assume that `factory` returns a value of the correct type if `type_` is specified per [33](https://github.com/chrispyles/fica/issues/33)
* Added the `required` argument to `fica.Key` per [13](https://github.com/chrispyles/fica/issues/13)
* Update `fica.Config` to support inheritance per [35](https://github.com/chrispyles/fica/issues/35)