Add ffmpeg helper as submodule for extract_srt_subtitles_to_files

[bambanah]

Update to include this PR:

• Enable calculating progress when no frame= found in ffmpeg progress message Josh5/unmanic.plugin.helpers.ffmpeg#5

Since extracting srt files doesn't include a frame value in the ffmpeg progress messages, we need to use the time value instead

Note: This will need to stay in a draft state until the PR is merged, and this PR will then need to be updated to point to the latest commit number

[Josh5]

@bambanah I've created a new project here for use to continue developing this plugin. Are you able to re-open this PR against it:
https://github.com/Unmanic/plugin.extract_srt_subtitles_to_files

[bambanah]

@Josh5 No worries! Just waiting on this PR to be merged so that this plugin can display progress. Will open PR against the new plugin repo after that