Add support for visibility property. Resolves #146.

[nsaunders]

Description of the change This pull request adds support for the visibility property. Mostly a direct port from Clay with the following caveats in mind:

• separate is not a valid value: https://github.com/sebastiaanvisser/clay/issues/225
• auto is not a valid value: https://github.com/sebastiaanvisser/clay/issues/224
• unset is a valid value: https://github.com/sebastiaanvisser/clay/issues/224

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Checklist:

• [x] Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• [x] Linked any existing issues or proposals that this pull request should close
• [ ] Updated or added relevant documentation in the README and/or documentation directory
• [x] Added a test for the contribution (if applicable)

[nsaunders]

Thanks for taking a look @thomashoneyman !