Make date parsing more robust

[jonasengelmann]

I am using zotero link and currently the date is missing for some of my entries or the year is wrong by one. I did some digging and noticed that a date entry of "1998" is parsed into {year: 1997, month: 12, day: 31}, and the entry "12/2006" cannot be parsed at all ( I never enter dates manually, so I did not opt for this format, it is probably the result of fetching by DOI or ISBN). I also looked into Zotero's API response and found that it already provides a parsed date (meta.parsedDate), "12/2006" is returned as "2006-06". (There is also a handy meta.creatorSummary field).

I tried to create a PR, but it would require restructuring some data handling, as currently only item.data is handled. @MunGell would you rather prefer rewriting the ZoteroItem to mirror what is returned by the Zotero API or just add some extra fields?