I was originally going to do this with a plugin - see #357 - but the more I work with metadata.json the more I want it to just accept YAML as an optional alternative to JSON.

The best example why is still this one: https://github.com/simonw/russian-ira-facebook-ads-datasette/blob/master/russian-ads-metadata.yaml

YAML is just SO much better than JSON for multi-line strings - in particular HTML and SQL, both of which are common in metadata.json files.