Multiple domains

[y-lohse]

As of now, we're only fetching ressources that are on the same domain as the base url. This probably means that anything on a CDN won't get downloaded, which can potentially break pages. This is also somewhat related to video downloads (#14).

On the other hand, we need to stay uite strict about what we download or we'll end up downloading advertising scripts or images, which we really don't want.