general: support for arXiv 5-digits IDs

* Amends the several regular expressions spread around the code that
  deal with arXiv identifiers so that the upcoming 5-digits based
  identifiers are also supported.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Merge pull request #59 from kaplun/arxiv-5-digits

general: support for arXiv 5-digits IDs