Fix head request checks with hashtag

Ran into an awful edge case just now.

`https://help.github.com/articles/searching-issues#no` currently passes because `https://help.github.com/articles/searching-issues` passes. That hash reference though, `#no`, should fail, because it does not exist on the page. 

This tweaks things a bit to perform a `GET` request on pages that have a `#` in the URL. Then, it calls Nokogiri to try and find that reference.

I'm not super pleased with the implementation because I think it will slow down the build a bit. But there's no alternative that I can see.

/cc @penibelst for review 
No, we always add it as part of a transform. But I guess I should check if the domain of the `href` is github.com, and then just do that check.