Create Cite element even if no matching reference in the biblio.

* Add ??? as fallback text for non-resolved citations.
* Biblio:  Put references (including a header at the end of
  the document, if one exists) inside a Div with class "references".
  This gives some control over styling of references, and allows
  scripts to manipulate them.
* Markdown writer:  Print markdown citation codes, and disable
  printing of references, if `citations` extension is enabled.

NOTE:  It would be good to improve what citeproc-hs does for
a nonexistent key.
Merge branch 'altcite'