viaplay: try harder when extracting video IDs

Before this change, the viaplay class would look for the video id only in the
URL. With this change, it will also look for it in the HTML document (after
fetching it) if it isn't in the URL. This makes svtplay-dl able to support
"latest episode" URLs, like:

    http://www.viasat4play.no/programmer/viva-la-bam

vs

    http://www.viasat4play.no/programmer/viva-la-bam/359853

Thanks to @avtobiff for reporting this issue.