RFE: check allowed authors

[scop]

A check for allowed authors would be nice, for example so that projects wanting committers to use their project/organization related e-mail addresses for commits could check that.

I think a regex config option and related (inverse) matching similar to ignore_author_re could be used for this.

[epage]

Mind writing up a proposed schema with an example of what data it would allow/disallow and what that message would look like?

[scop]

Mini-PR in #354 -- feel free to reuse that and the associated branch for the implementation if you're taking this on.

• The regex would be matched against the commit author (whole string), just like ignore_author_re.
• If the regex matches, all good, no messages.
• If it does not match, would cause an error with a message like

  Commit author `{}` does not match allowed regular expression `{}`
  

[epage]

Seems reasonable if someone wants to implement it.