CLI: Add push tag flow to creating the gb release

[jhnstn]

Fixes #173 Depends on changes from https://github.com/wordpress-mobile/release-toolkit-gutenberg-mobile/pull/188

This adds a release tag to the Gutenberg repo after the PR is created.

Testing:

• Run the release prepare gb {version} command against a fork of Gutenberg
• Note the "Adding release tag" message after confirming the PR creation
• Verify that the tag is added to the remote repo.