BibFormat: recjson update using BibReformat

* Adds `recjson` to the list of available formats that BibReformat
  could use. (closes #1708)

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
People would expect a `CFG_...` to live either in global `invenio.conf`, or in local `bibformat_config.py`.  Perhasp we should move it to the latter?  Or call it `_CFG_...` with leading underscore to indicate that this is a very internal beast?