Allow translators to define hidden preferences

This goes along with zotero/zotero#223

I don't think there's a way for users to define these preferences atm with Connectors or Bookmarklet (I saw that preferences are stored in HTML5 localStorage, but I don't think we provide any way to edit it). Either way, this will at least make the default hiddenPrefs values available to translators.

Perhaps we should be fetching preferences from Zotero Standalone in Connectors (and bookmarklet?) It's a bit of an overhead, though communication over loopback interface should be fairly fast.

Also, there are no `__defineGetter__` declarations for displayOptions or configOptions. I get that displayOptions is not used in connectors, but I thought configOptions would be. Do we not need to deepCopy these in connectors? Let me know if we do, then I'll add it in.