Config to set additionalProperties=false by default

[dsample]

We use TV4 in our schema tests, and in order to improve our tests, I'd like to be able to tell my tests to treat every schema as having additionalProperties: false, so that at least during tests there will be errors flagging when test data doesn't strictly conform to the schema.

In addition to that, it would be nice to be able to write a test to effectively 'lint' the schemas, and flag when we have schemas which don't have an additionalProperties setting at all. I could write something to read through the schema as JSON, but it would be cool if it were already possible.