ext: CrossRef Flask extension

* Queries DOI record data from the database or CrossRef as JSON.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
ext: CrossRef Flask extension tests

* Adds tests for DOI found and not found.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>