Add support for collections

Closes https://github.com/jekyll/jekyll-redirect-from/issues/36.

I'm pretty amazed that we only need three lines of code to do this. Of course, I could be making an obvious mistake somewhere, too. :wink:

/cc @benbalter and @parkr for review
Going along with my comment below, it would be easier to use `site.docs_to_write` here.