[Youtube] support for video watched percentage

[repaxan]

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Description of your issue, suggested solution and other information

Partially watched youtube videos have a corresponding partially filled progress bar on the thumbnail, in the youtube UI. Currently the mark-watched does not have proper support for this and you get a very slightly filled progress bar.

It would be good to have a way to mark it as 100% watched progress, either with the default behavior of --mark-watched or through some new --mark-completely-watched flag.