patternProperties -> x-patternProperties

[jcts3]

Proposed fix for #17.

Mimicked the behaviour of what's done here to convert the patternProperties object into x-patternProperties.

Given it's only using the Specification Extension functionality of adding ^x- on the front of the unsupported property, it's not supported by a lot of tools, but it abides by the specification at least?

I haven't been able to contribute much to Open Source before, so I hope this is helpful!

[JaredCE]

Hey. Thanks for this.

I must admit i'm at a slight crossroads about including this. I had thought about doing it myself, but my concern is that once you start renaming properties to ^x-, you lose the usefulness of the schema unless you have a tool that then re-converts ^x- properties back to their original state.

Ideally the world will move on to OpenAPI 3.1.X which allows for JSON schemas greater than Draft-04 and there won't be a need for this library.

Will have a think, but thank you for the PR and please don't be dissuaded from adding anymore additions or improvements.