Notify User if the talk page exists

Upon typing a title, the plugin will check for the main page, the talk
page, a draft page in the format 'draft:title' and the talk page for the
draft, in that order of importance

Known issues: The error message flashes a little when the name is being
typed, due to the different namespaces being checked.

http://www.google-melange.com/gci/task/view/google/gci2014/5877487489777664
https://github.com/WPAFC/afch-rewrite/issues/26
This is not actually an API action=query parameter. See the general query documentation: https://www.mediawiki.org/wiki/API:Query

Essentially, we'll just need to do a basic action=query request. The title can just be `'Talk:' + page.rawTitle`. Thanks for all your work with this!