refs: https://github.com/TryGhost/Toolbox/issues/440
New command to generate demo data, creates data for over 20 tables in
Ghost, suitable for testing most features of the dashboard, as well as
making guided product tours using newsletters, tiers, many posts and
tags.
Usage: `yarn start generate-data`
Optionally, keep your existing posts / tags with: `yarn start generate-data --use-existing-tags --use-existing-posts`
