arxiv API support

[ksilo]

Hey, trying to make a contribution to this project. Will be happy if you could review and make suggestions how can I improve it.

[stijnh]

Thanks! This is really great work, thanks for your contribution. Support for ArXiv is indeed a major contribution for the project. I am happy to see the work that you put into this.

I have reviewed your pull requests, see the code review.

Here are some general points:

• Add search_arxiv to sources/__init__.py so its available directly as litstudy.search_arxiv
• The dependency feedparser should be added to setup.cfg
• The Document class has bunch of methods that can be overriden. Do you think there are more that are provided by the ArXiv API? (Maybe abstract, keywords, or language)?

[stijnh]

Thanks for the work! Looks good! I'll merge this and it'll be part of the next release of litstudy.