Add in back-compat shim for existing cli argumaent form

This is currently used in this form via .kitchen.yaml (for instance <https://github.com/ministryofjustice/metrics-formula/blob/d87a59f8b7e86378e0a310c8b47e5393e131d3d2/.kitchen.yml#L25>) and having this shim makes the upgrade path a lot easier.