Add a new option in WSD2Java to skip the generate the type classes from schema

In some cases where the schemas are not inluded in all wsdl's directly or indirectly it's painful to have separate build sections in plugin or wsdl2java commandlines since xjc would make codegen fail if it doesn't find the schmea on which customization is to be applied is not part of any direct/indirect import in any wsdl when one specify wsdl's with wildcard or filter.
If the codegen plugin can give option to specify disabling code generation for schemas it will be helpful to have more control over code generation and jaxb customization on schema.