Add support for applying set of resources via kubectl

Discussion: https://github.com/GoogleCloudPlatform/kubernetes/issues/1905
Could we unmarshal directly into `[]runtime.RawExtension` and avoid the wrapper object altogether in the serialized form? I think this would be easier for users to simply concatenate several objects into a single file, as well as easier for simple config generators. The default api version could be set using profiles (#1755), or in a configuration generator, or in a simple transformation pass.