Enable using cli in automated ci pipeline.

[mauricedoepke]

I want to use the gocardless cli to enable automated tests of our payment logic in our ci pipeline.

Is your feature request related to a problem? Please describe.
Currently the only way to login with the cli is by using oauth (opening a browser link). This requires manual interaction or a unneccessarily complex browser automation to automatically open the oauth url via script.

Describe the solution you'd like
It would be nice to be able to alternatively authorize the cli by using an api key. Stripe e.g. does it like this:

stripe listen --api-key ${STRIPE_TEST_API_KEY} --forward-to http://localhost/webhook

[george-karma]

Bump