CLI: add info about unused schemas in the validate command

Created by: jmini

With #74, the method ModelUtils.getUnusedSchemas(OpenAPI) was updated. It computes the list of unused schema. We could append this information to the output of the validate task. It is a kind of information that something is not right in the schema.

What do you think?

cc: @ackintosh (you seems to use this feature, are you interested?)