WEBM container support on Remux Video Files

[Josh5]

Add WEBM support to video_remuxer Plugin.

Before this is done, we need the other Plugins created:
Issue #35
Issue #36
Issue #37

[Josh5]

Previously raised as an issue for Unmanic:

It would be nice to see WEBM supported since that is open source and patent free. It means that you would need to support VP8 and VP9 and AV1. You would also need to support Opus and Vorbis audio formats. All subtitles would be to be converted to WebVVT as well. All of this can be easily done with ffmpeg.

https://en.wikipedia.org/wiki/WebM
https://developer.mozilla.org/en-US/docs/Web/Media/Formats/Containers#webm
https://developer.mozilla.org/en-US/docs/Web/API/WebVTT_API