Simplify

Made minor simplifications to the internal API for Configuration.

* Use `yield` instead of `Configuration#call`. `#call` should only be defined for objects that represent code, imho.
* Removed `Configuration.build` in favor of allowing `Configuration#initialize` to initialize it's instance variables and yielding itself. Yielding from within `#initialize` and having `.new` return the new instance is the same behavior as `.build`.