Add org-ref like cross-reference support

[bdarcus]

Citar will always focus exclusively on citations, and cross-references are out of scope; a different kind of feature, with different UI/UX, code, etc.

But thought I'd add this here for the record.

I have, however, been playing a bit with a separate minimal alternative, just to understand if there's promise in some smaller changes to org to accommodate this functionality better, or a small package that does similar.

https://github.com/bdarcus/oxr

I'm unsure I'll ever formerly release it, as I don't have great need for it myself, and so don't understand the details as well as I do citations. It's also very simple, which is largely the point.