new theme should have a --format flag

[cgalo5758]

This is minor, but when creating a new theme, I'd like to use another format other than toml for archetypes, config, etc.

[jmooring]

archetypes, config, etc.

To clarify, there is one archetype file and one configuration file. I'm not aware of any "etc" unless you're referring to the placeholder content, which should be deleted anyway.

[cgalo5758]

Correct. Placeholder content too.