RFC: Check signoffs for an email that matches commit author or committer email

[robbkidd]

In an effort to confirm that the author or committer of a commit is the one who signed off a commit, we should only pass a DCO check if the email address for the commit author or committer appears in a sign-off line.

[robbkidd]

This would also catch cases like #17 where someone has copy'n'pasted the example DCO sign-off into their commit message without understanding that they should substitute their own contact information.