https://github.com/opencontainers/image-spec/pull/371 added in the labels field to [this doc](https://github.com/opencontainers/image-spec/blob/master/config.md#properties), but didn't update the struct exported by this repo ([here](https://github.com/opencontainers/image-spec/blob/master/specs-go/v1/config.go#L18)) to include the new field. It should be added.
