Validate `Config`

It would be nice to provide a `json-schema` to install locally for validation. It might be tricky to combine the schemas from the plugins, but in principle it should be possible with [`$ref`](https://stackoverflow.com/questions/37154317/merge-properties-of-one-json-schema-in-another-json-schema) (requires [upstream PR](https://github.com/python-jsonschema/jsonschema/pull/1049)).