Refs: https://github.com/joyeecheung/node-core-utils/pull/72#pullrequestreview-74269096

If the tool generating the metadata is not sure about something, it can put a ? before the metadata, and let core-validate-commit complain. For example, if someone LGTM via comments, the tool can generate the reviewer as:

?Reviewed-By: someone <mail@email.com>