walker should print yaml output

walker now takes option(--yaml) to print yaml output.

By default, it will continue printing json(for existing users).

i could not add any tests for this, because that would need cucumber setup for methadone. Ideally we should have setup cucumber as part of https://github.com/alphagov/vcloud-walker/pull/37.  

I will raise a chore to setup cucumber for testing walker cli.