0.5.0 optionally transform x-extensible-enum to enum

• Adds the CLI option --preserve-x-extensible-enum (default: false)

  If you apply Zalando RESTful API and Event Guidelines, you are not using enum in the API, but x-extensible-enum. This is great for documentation, but it hinders development a little. Therefore, the output contains enum instead of x-extensible-enum. This affects the types as well as request/response validation. You can keep x-extensible-enum which allows for any string to be used by passing the --preserve-x-extensible-enum parameter.