Add option to prefer hw-accelerated codecs

[07151129]

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It doesn't look like ffmpeg CLI driver is able to provide the list of hw-decoded codecs for current platform, so I suggest hardcoding it within youtube-dl, and matching the format when trying to load a video.

[dstftw]

Use format selection.

[07151129]

Thanks; I did not realise format strings have become so powerful.