Configuration module builder

Removes the global pollution of class-level configuration while maintaining backwards compatibility with the previous style.

* A configuration instance is used at the `Virtus` level
* `Attribute.coerce` is deprecated and proxies the value to `Virtus.coerce`
* Added `ModuleBuilder` used by `Virtus.module` to allow for Virtus module creation
* Configuration is injected into the module via an included hook by overriding the `.attribute` method with custom configuration options.