Add support for getting a changelog line from a PR/Issue comment

[wagoodman]

What would you like to be added:
Be able to read a changelog line to be included in the changelog from the contents of a comment. This could work in a similar way as changelog-ignore labels, where if a changelog-comment label is added then chronicle would look through the comments for a trigger word/command and incorporate the user-provided changelog line.

Why is this needed:
This would have been great for anchore/syft#1696 where attribution should be given to multiple users, include multiple PRs, and have an odd linking back to a single issue. This might be able to be handled by chronicle automatically, but it would be great to be able to have an override on the PR directly and not think about it.