Extend JSON Schema method

[samlown]

Jotting down this idea... there are a lot of requests to be able to support more advanced definitions which start getting really complex inside Go tags. The current JSONSchema() method helps support these cases, but has the tradeoff that you need to define the whole model.

A possible alternative could be to support an JSONSchemaExt(base *jsonschema.Schema) method that will generate the base schema for the Type as much as it can, then allow the type to override and add any additional fields, e.g.:

func (User) JSONSchemaExt(base *jsonschema.Schema) {
  base.AnyOf = &jsonschema.Schema{.....}
}

[pawndev]

Just wrote a simple version of what you said: #52