Allow more than doi (arxiv ID) / manual entry of paper

[nicebread]

arxiv does not provides dois

[alpkaanaksu]

I think we discussed about this before (after you created the issue).

We decided to automatically detect arXiv IDs. So instead of making it completely manual, we'll also support arXiv.

[nicebread]

But I think there should also be a fallback solution, where users can enter title, year, and some sort of link manually.