Add `--visibility` flag

Allow consumers to update only `public` or `private`, or `all` (default) repos to use the new branch by default.

Example usage:

```
github-default-branch --org my-org-name --visibility public
```

Start from the CLI entry point for the github-default-branch command and trace how repositories are selected before their default branches are updated. Add the --visibility option so public, private, or all repositories are selected, then verify the example command and the default all behavior.

Written by the indexing model from the issue text.