Convert relative URLs in feed contents into absolute ones

When feed item body has images/videos with relative URLs, currently they cause requests to the server where the app is hosted, resulting in 404s and sometimes 302s (rkh/rack-protection#56).

I've seen an opinion that such feeds are not valid, but Google Reader supports that, so I think you should, too. Example: https://www.google.com/reader/view/#stream/feed/http://nakkaya.com/rss-feed