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
yeah, I think overall I'd rather not have the en-dash if we just have one of the two. Unless @zuphilip you had a particular scenario in mind that we're overlooking?