Introduce YAML support for cli

[Cory Todd]

It would be cool to have built-in support for YAML on the CLI. Most of my data is YAML and I end
up having to pipe it through yq so native would be nice.

Since this only affects the cli, I propose leaving all of the current structure alone. We would need a new loader/dumper pair and a --format parameter so the user can hint their file type. We could get
fancy and auto-detect the type but I'm not keen on that yet.

There should also be explicit tests for all of the loader/dumper variants.