Add footnotes.

[jbeder]

This relies on the footnote extension in cmark, which does not expose footnote type names, but does expose enough information via the type enum.

Bug/issue #, if applicable:

Summary

Adds new footnote types FootnoteDefinition and FootnoteReference to the markdown tree.

Dependencies

None. It would be nice to use swiftlang/swift-cmark#57, but it's not necessary; there's a small hack that can reliably detect when cmake produces footnotes.

This is heavily based on #129, which also adds footnote support; however, that PR does rely on the upstream swift-cmark PR, whereas this one does not.

Testing

New test introduced to verify the parsing of footnotes, which pass with ./bin/test.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary