Attempt to directly proxy or redirect video and audio content

Currently we block certain sites which have high video content, but not others. A better solution would be to see if we can either:

* Detect media content by mime-type
* Issue a redirect to the source

Or

* Maintain a list of domains and intercept them in `wombat.js` on the client side, so they are never attemped to be proxied in the first place

The first is good, as it works without any manual intervention to build up a list of domains. The second is good because it doesn't require many any connection to the upstream service.

It's likely that we will see cross domain issues with this.