refextract: recognize system identifiers

[annetteholtkamp]

Sometimes a url contains a system identifier, e.g.
https://cds.cern.ch/record/2064383
in https://inspirehep.net/record/1611588
refextract could isolate the system id and put it with appropriate prefix into a designated field.
Create a new field for external identifiers? Or put it into the same field as a DOI?
This system id can then be used to link the reference to a record.
We have more than 3000 records with a CDS url in the refs.

Similarly for ADS (~1000 records):
e.g. http://adsabs.harvard.edu/abs/1990ApJ...360..242S