update: remove ruamel requirement, and improve '--create-metadata'

If ruamel.yaml is not available, this will fallback to using PyYAML. This also adds some blank fields to the newly created template to make it easy for human editors to fill in. RE: !299 (comment 34281555)

closes #343 (closed)

@IzzySoft, for the additional fields you want to add to the template, I think you should manage it in a git commit in your own repo for now. If other people want to have a configurable template, then I'd be happy to accept your merge request with it. I just want to avoid adding features for one person.