[Feat] Auto Generate Thumbnail while Uploadin

[ar5hil]

Proposal

tdl up --thumbs --thumbs useed for thumbnails; no means no thumbnail; auto means auto generate; true pick them from folder; advanced with exact timestamps

Background

whenever i upload a video it uploads as pitch black with no thumbnail or it shows the first frame of the video after loading

Workarounds

i create some thumbnail with ffmpeg at any timestamp in .png then rename them to .thumb then while using tdl up, it works

[AKL55]

.thumb doesn't seem to always work, Maybe it's a codec/length/size thing i don't know. Would be nice to be improved since it's a really useful feature.