SwagEdit should validate x-example and x-examples property values

[andylowry]

Though it's not formally part of the Swagger 2 spec, x-example has been implemented in swagger.io projects as a standard vendor extension applicable to serializable parameters.

While vendor extensions in general are not constrained in terms of their values, x-example must be a scalar value; otherwise, SwaggerParser will fail to parse the model.

Similarly, x-examples is recognized by swagger.io projects as a means of providing examples for body parameters, in the form of a map keyed by mime types. In this case, arbitrary values are allowed as example values, but the overall x-examples property value must be a JSON object.

It would be good if these requirements were understood by SwagEdit.