Search identifiers for citations missing one

[nemobis]

While citation bot on the English Wikipedia attempts to convert citations to structured format for {{cite journal}} (when they already have a DOI or such), there's nothing of the sort for ISBN and most wikis have no such bots at all.

Even if it would make the process considerably slower, and would probably depend on #10 , it would be nice to query additional services and add more identifiers:

• CrossRef offers https://www.crossref.org/blog/reference-matching-for-real-this-time/ for DOIs (even unstructured text is fine, but then it's better to check the initial citation actually had the same title or something);
• OpenLibrary.org or isbndb.com might be used for ISBNs.