Changelog checker should take a `release_candidate` or `rc` flag that checks for entries to be placed in given version rather than Unreleased

[mcmire]

As stated, if you make a release PR in a repo, then the changelog checker will fail even if the release PR makes changes to package.json and you've documented those changes in the changelog. This is because it's looking for those entries to show up in Unreleased rather than the version being released (as can be seen here).