Support document types that don't have DOIs
At the moment, there is a hard-coded dependence for the flat-file database on the document having a DOI.
There are (at least) two cases that this won't work for:
- books (ISBN)
- unpublished, but citable articles (arxiv, etc...)
In addition, it should be possible to cite other things that aren't academic articles/documents with a DOI, such as:
- Board games
- Video games
There are also things that have DOIs that aren't papers (datasets, software, etc...) that we should deal with as well.