feat: Add SOURCES.toml for dataset metadata

- Add SOURCES.toml to provide supplemental (extrinisic) metadata on datasets, from SOURCES.md, in a form usable by build_datapackage.py
- Include resource descriptions, sources, and licenses to supplement script output
- Preserve existing markdown content for future documentation
- TODO: Remove duplicated content between descriptions and sources
- TODO: Incorporate resource-level column descriptions into table schema, where available
- TODO: determine if root-level $schema property should be specified in the TOML file with the value "https://datapackage.org/profiles/2.0/datapackage.json" per Frictionless Data guidelines

Resolves vega#634