Display additionalProperties and other keywords

[duncandewhurst]

Related to OpenDataServices/sphinxcontrib-opendataservices-jsonschema#48, docson doesn't display the additionalProperties keyword. It would be good to show that so that users of the schema browser have access to it.

Similarly, I think we should check which other keywords are supported by docson and add any that are used in our standards. We could consider a flexible approach like the one suggested in the link above.