BibCheck: $$9 bibcheck to dois

* By default, adds $$9 subfield with "bibcheck" value when adding DOIs
  in the doi.py BibCheck plugin.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
Reviewed-by: Samuele Kaplun <samuele.kaplun@cern.ch>