BibFormat: new bfe_date parsing/formatting options

• Adds parameters to customize the parsing of the date (format, source). Source format can also be guessed provided that mx.DateTime module is installed.

• Special output of year-only dates.

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>
Tested-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>