Enable hostname prepend for video thumbnails

[BerciTheBeast]

What problem does this feature solve?

Video thumbnail_loc properties are currently not being prepended with hostnames if the URL is not valid.

What does the proposed changes look like?

If the thumbnail_loc is not a valid URL it would be prepended with the hostname, similar to image URLs. It would be nice if this option was toggleable in the config or even per video entry.

_{This feature request is available on Nuxt community (#c97)}