Added AbsoluteSourceFilter for changing relative image src to fully qualified

This is useful if an image is root relative but should really be going through a CDN, or if the content for the page assumes the host is known i.e. scraped webpages and some RSS feeds.

Tests are in there too.

Let me know if you need anything else, or if you don't think this would be useful for others no worries.

Thanks,

Ben