Added an allow_null option which adds the 'null' type to all properties in the schema

Added an allow_null option which adds the 'null' type to all properties in the schema. This is useful when you need the schema to allow null values in the JSON being validated.

This is a bit of a hack so I understand if you don't want to merge the changes. We've just picked up Pacto today and this null values issue is blocking us. Happy to hear your feedback if you have any suggestions.