Cnxml

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- Add support for new optional metadata (28)

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- Relax required information when parsing metadata (25)

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- Add tests for cnxml <md:derived-from ...> validation
- Fix failing derived-from cnxml validation tests
- Use a self-closing md:derived-from as the valid repr
- Move the metadata parsing from
`Press <https://github.com/openstax/cnx-press>`_ to cnxml (#20)
- Add a CLI utility for extracting cnxml metadata to json (22)

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- Extend validation API to accept multiple documents to validate

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- Tweak mdml schema to properly allow any metadata element under derived-from