Add displayOptions parameter to the items endpoint

I needed this for my ever-expanding zotxt-autoexport bib-file from zotero solution. I hava also submitted a merge request for zotxt-emacs.

I don't know if I have implemented it in the absolutely best way, but it seemed quite straightforward to me, however, javascript is really not my first language.

Feel free to merge if you think it would be a useful addition (otherwise I will keep my own forks).