update --ugoira webm to use the same FFmpeg args as Danbooru
--ugoira-conv -> --ugoira vp8
--ugoira-conv-lossless -> --ugoira vp9-lossless
--ugoira-conv-copy -> --ugoira copy
(--ugoira-conv and co still work as before,
but --help now lists only --ugoira)