Add "Remux movie fragments to MP4 when possible" to fix ExifTool's not being able to handle movie fragments.

[ProtagNeptune]

[08:35:09.365][Info] Loading image from https://cdn.donmai.us/original/3b/85/3b85d1b224e33116992e3317e7a10291.mp4 1
[08:35:09.377][Info] Not enough information to directly load the image (from blacklist: 0 / from file url: 0 / from filename tags: 1/2)
[08:35:09.378][Info] Loading image details from https://danbooru.donmai.us/posts/6951211
[08:35:09.739][Info] Loading and saving image from https://cdn.donmai.us/original/3b/85/__hoshino_ruby_oshi_no_ko_drawn_by_delalicious3_voice_actor_and_shiina_ecchi__3b85d1b224e33116992e3317e7a10291.mp4 in C:\Users\jelly\Grabber\#Monitors\OshiNoKo\vid\3b85d1b224e33116992e3317e7a10291.mp4
[08:35:10.324][Error] [Exiftool] Error: Can't yet handle movie fragments when writing - C:/Users/jelly/Grabber/#Monitors/OshiNoKo/vid/3b85d1b224e33116992e3317e7a10291.mp4

Error: Can't yet handle movie fragments when writing - ExifTool

Proposed Fix

1. If Grabber get the error: Can't yet handle movie fragments when writing.
2. Then Remux the video: ffmpeg -i input.mp4 -c copy output.mp4.
3. And then change the metadata with ExifTool normally.

[ProtagNeptune]

Update 19dcd19

[02:23:08.547][Info] Loading image details from https://danbooru.donmai.us/posts/6951211
[02:23:08.847][Info] Loading and saving image from https://cdn.donmai.us/original/3b/85/__hoshino_ruby_oshi_no_ko_drawn_by_delalicious3_voice_actor_and_shiina_ecchi__3b85d1b224e33116992e3317e7a10291.mp4 in [C:\Users\jelly\Grabber\#Downloads\vid\3b85d1b224e33116992e3317e7a10291.mp4](file:///C:/Users/jelly/Grabber/#Downloads\vid\3b85d1b224e33116992e3317e7a10291.mp4)
[02:23:09.406][Info] Invalid file extension (mp4 to ) for [C:\Users\jelly\Grabber\#Downloads\vid\3b85d1b224e33116992e3317e7a10291.mp4](file:///C:/Users/jelly/Grabber/#Downloads\vid\3b85d1b224e33116992e3317e7a10291.mp4)

The file end-up named 3b85d1b224e33116992e3317e7a10291 with no extension at all.