PARTIAL FIX EXT-3491: only bring up "external browser"notification if target...

PARTIAL FIX EXT-3491: only bring up "external browser"notification if target is external or pref is set
Review #86
This changes the logic to bring up the notification "Do you want to open your Web browser to view this content?" only if either the target is _external, or the user pref says to. Otherwise it avoids the notification.