Add Post-bump callback

[passsy]

Besides the version in pubspec.yaml, I also want to bump the version in my README.md. It would be great if the BumpVersionCommand would provide a callback allowing me to add my custom logic

..addCommand(
  BuildVersionCommand(
    afterBump: (oldVersion, newVersion) {
      final readme = SidekickContext.projectRoot.file('README.md');
      readme.replaceFirst(oldVersion, newVersion);
    }
  ),
);

It's unclear to me how to connect it with --commit because it doesn't know about my changed file. Most likely it doesn't work out