Add --dry-run option

[ecraig12345]

Add a --dry-run option (intended for the CLI only) which does the following:

1. Makes all changes locally
2. Runs npm publish --dry-run (skipped if the publish option is disabled)
3. Commits the changes locally and merges them into the target branch, but does not tag or push (skipped if the bump or push option is disabled)
4. Stays on the current branch (and doesn't delete the publish branch) so you can inspect changes

Fixes #503