Add a command to add a new package to a release

[mcmire]

Sometimes you want to add a new package to a release after you create a release.

We could add a new command to create-release-branch, perhaps add-package, that would allow you to:

• Bump the version of a package (specifying major, minor, patch, or a version)
• Create a new section in the changelog for the new version
• (Optionally) Populate the "Uncategorized" subsection with changelog entries

This would be an alternate to #28 (which is more about being able to run create-release-branch and edit the release spec or open the UI view again).