Support Ref IDs

[harniman]

See https://support.brightcove.com/en/video-cloud/docs/using-reference-ids
Sometimes it is better to refer to a video via its refid rather than the video_id. This allows the video to be re-ingested and re-encoded without having to update the embed code.
It would seem that if the value of video_id in the embed code is manually updated to the "ref:***" format that "public static function sanitize_id( $numeric_string ) {}" function strips non numeric values resulting in an empty video_id being returned.
Additionally the user should be able to choose to embed video_id or ref_id