Add (optional) processing step to disable additionalProperties #16
Description
In most circumstances it's not preferable to allow additional (unknown) string-indexed keys on your schemas. Currently, sometimes, those can pop up because of processing that leaves "type": "object" without additionalProperties: false.
It would be nice to have an option to blanket enable additionalProperties, always. This would also be helpful, because the default for additionalProperties is true when left blank.