Enable creation of the Publication by arXiv identifier

[cthoyt]

This PR mainly adds the function wikidataintegrator.wdi_helpers.publication.arxiv_api_to_publication and integrates it with the wikidataintegrator.wdi_helpers.publication.Publication class to allow specifying the source/idtype as arxiv.

I ran the code

python -m wikidataintegrator.wdi_helpers.publication 2006.08589 --source arxiv --idtype arxiv

and was happy greeted by

Successfully logged in as Cthoyt
2006.08589,Q96335314,,True

and my shiny new WikiData entry https://www.wikidata.org/wiki/Q96335314.

This PR adds a few minor tweaks as well:

• Enables specification of username/password from the CLI using getpass if it's not available in the environment or a local.py file
• Fixes unsafe usage of a mutable default argument for authors in Publication.__init__.
• Encapsulate CLI in wikidataintegrator.wdi_helpers.publication in main()
• Fix the definition of package metadata in setup.cfg
• Add vanity CLI, so you could run wikidataintegrator-publication 2006.08589 --source arxiv --idtype arxiv instead