Update Primo.js

Improve handling for pages, see https://forums.zotero.org/discussion/33862/princeton-primo-translator/#Comment_200487 , in the following order
 1. prefers spage, epage if both are given
 2. prefers overallPage if only one of spage or epage or none are given
 3. will use just spage, or just epage if this is only given
I'm wondering about this--in a way that makes sense, at least if this is really just the end page. But could we have buggy data with the whole page range in one of these tags? In that case it'd be better to add not include the en-dash. Speaking of en-dash:
shouldn't this be '–' rather than '-' It doesn't matter much since citeproc handles this in citations, but if we're doing this anyway we might as well do the right thing.