NF: `onyo show` should allow dumping YAML in dot notation

[aqw]

YAML in dot notation can be read in by onyo new et al. But Onyo is not capable of generating it.

The density and readability of dot notation is a matter of taste. However, the key feature I see is being able to use YAML-unaware unix tools such as sed, grep, etc without needing to use yq.

It's a nice-to-have.

The limitation is in dict_to_yaml(). The feature would then need to be exposed by Item/ItemSpec.yaml() and then onyo_show().