Add support for CITATION.cff format

[c-martinez]

This PR addresses #1469

I am not very experienced with R, but am a big supporter of CITATION.cff and encourage its adoption at every opportunity, so this looked like a good opportunity to contribute. I've asked a colleague (@PabRod) to review my code before sending this PR.

I don't think this is perfect, but I think it works and am happy to improve it as necessary.

[PabRod]

Some context

Citation File Format (CFF) files are plain text files containing citation info in both a machine- and human-readable manner. This format is endorsed, among others, by:

• GitHub, where it even renders a citation side menu (see more here)
• Zenodo, where it's used to autocomplete the repository fields
• Zotero

All context

Want to know even more? Please visit the Citation File Format website.