Add a yaml linting subcommand

[a-alveyblanc]

Replacement for running yamllint from the command-line, particularly in the case of CI.

• Expands yaml files (i.e. expands any Jinja found in yaml files), then performs linting
• Config file defaults to ./.yamllint
• Requires a directory to be supplied (could change this to run in the repo root?)

LMK what you think! Seems sort of barebones, maybe I'm missing some things.

EDIT: Just realized this probably needs a test :sweat_smile:

[a-alveyblanc]

Just added two simple test cases. This is GTG from my end @inducer

[inducer]

That test did not do much. FTFY, see pushed commit(s).

[inducer]

Thanks!