Request/Response Mocks Defined As OpenAPI Examples

[evan111]

Issue
A present Open API 3.0 Examples are returned randomly, the only differentiator is setting a Preferred HTTP response code:

         // Choose a random example to return.
	keys := make([]string, 0, len(mt.Examples))
	for k := range mt.Examples {
		keys = append(keys, k)
	}
	selected := keys[rand.Intn(len(keys))]
	return mt.Examples[selected].Value.Value, nil

Request
It would be great if we could define which mocks to return also using path params, query strings, and even request/response payloads. See how Microcks uses this ability: http://microcks.github.io/using/openapi/

[danielgtaylor]

See also #21.