add implementation of https://forums.zotero.org/discussion/3673/2/original-date-of-publication/ for #237
add implementation of https://forums.zotero.org/discussion/3673/2/original-date-of-publication/ for #237

egh/zotero-better-bibtex,maxxk/zotero-better-bibtex,davepwsmith/zotero-better-bibtex,gracile-fr/zotero-better-bibtex,egh/zotero-better-bibtex,maxxk/zotero-better-bibtex,davepwsmith/zotero-better-bibtex,gracile-fr/zotero-better-bibtex,maxxk/zotero-better-bibtex,egh/zotero-better-bibtex,egh/zotero-better-bibtex