Add option to set biblography language in the Document Preferences window

[tjowens]

We have had several requests for the ability to set the language localizations on a per document basis. The Doc prefs window seems like the most natural place to do this.

See ​http://forums.zotero.org/discussion/2817/

[dstillman]

Also mentioned on #800. A document-specific locale could either default to the global setting or, probably better, the last-used document locale.

[simon]

Frank, should I modify the style on the fly to handle this (and potentially also the ibid and title casing options in #1088), or do you think it's better to have explicit configuration options in citeproc-js?

[fbennett]

I've implemented this in the multilingual branch. It's done with processor options.