Adding extra user-defined query (partial fix to do cmcontinue)

[nsunami]

I had the same issue with #27 on pulling more than 500 responses. I added extra_query argument to pages_in_category() so that I can pass cmcontinue to queries. This is not a full fix for #27, but at least I can perform multiple pages_in_category() to use cmcotinue. Hope this helps a bit!