Automatically fetch newer version from ArXiv

[lazymike]

..or alert the user of new versions available, e.g., when the pdf of the article is opened.

[alfureu]

Would it be possible to extend this to other services as well? It would be great if we could download the PDFs automatically/seamlessly in the background. There are also lists for EZProxy access, if one is affiliated with an institution, to download PDFs automatically: https://ezproxy-db.appspot.com/