Add a option for null values to be accepted in validations

This change allows for a single field to be null (in addition to
its other types). This is different than required in that you
can actively change a value to None instead of omitting or
ignoring it. It is essentially the ignore_none_values, allowing
for more fine grained control down to the field level.
Docstring 'versionadded' for 'nullable' rule
Documentation: 'nullable' rule
Changelog: 'nullable' rule
Merge branch 'KalebPomeroy-add_nullable_type_to_fields'