Add support for patternProperties in schemas

JSON Schema draft 4 has patternProperties which is not supported by Swagger 2.0. patternProperties allows to have loosely defined objects where the list of keys is not fixed in the spec (like additionalProperties allows in Swagger 2.0), but the format of the keys is restricted with a regexp (contrary to additionalProperties where keys are not at all limited).

Here is a use case from #380:

name: pop
description: population by country
type: object
patternProperties:
  '^[A-Z]{2}$':
    type: integer
    description: the population of the country indicated by the property name.

[webron]

No, patternProperties did not make it into 3.0.

[hotgazpacho]

So, how can we help get patternProperties into the OAS spec? The lack of this is really hampering our ability to express our response objects in OAS.

Use case:

We have an API that allows a customer to ask “what do you know about these things?”, where things can be of different types. We use the values passed in as the keys in the response to say “here’s what we know about this item”.