[#64583408] Temp cli

This PR adds a CLI. It only allows you to configure - doesn't do diffing yet.

The plan was to add the CLI properly, as per notes here https://www.pivotaltracker.com/story/show/65370914, and I made a start in another branch, but I ran out of time to get that finished. In order not to hold up the testing taking place next week I'm committing an interim version - this is basically in the exact same form as the rest of our tools.