[BibTeX] Protect words with initial uppercase letters on export

Introduces extensions.zotero.translators.BibTeX.export.dontProtectInitialCase hidden preference. If set to `true`, will (mostly*) restore previous behavior of only protecting words with internal upper case letters.
*previously all fields except for "pages" were protected. With this patch only a select list of fields gets protected.
Supersedes #477
Depends on zotero/zotero@eb7959a11dc1970804a10af7d6055ba351e0c6be (zotero/zotero#583)
If someone can think of a non-messy way to avoid using a callback function for this (i.e. some witty regexp magic), that would get rid of Zotero dependency.