feature request: support new syntax for uncertain metadata

[joyeecheung]

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>

[joyeecheung]

cc @nodejs/automation see if anyone is interested in picking this one up. This would need an update in lib/rules/*.js and an update in https://github.com/git-lint/gitlint-parser-node as well (because that's where the context comes from)

[riyadhalnur]

I'll pick it up.