New features:
1. Added the `enabled` Field property, which defines whether a given Field object is enabled or not. Disabled fields aren't taken into account during validation. This property may be a static attribute, a callable, or an actual Python property.
2. A Field's `__init__` method may now be used to set attributes, just as you would do with any other Python object. You may then use those attributes normally in the `validate()` method, since they now belong to the field's instance.