internal/cmd/cmd.go: Synchronize streams to end of first silence

[howlett]

Detect silence by use of ffmpeg on the english language stream and substitute $seek with the value. This allows for approximate synchronization of video streams.

Still requires documentation update.

Signed-off-by: Liam R. Howlett [email protected]

[hrafaelveloso]

This looks promising, and can at least make the differences between streams smaller and easier to sync after that. This makes it possible to use the $seek variable to use with the "--start" command on mpv as pointed out on a reddit thread: https://www.reddit.com/r/F1TV/comments/tggpe8/i_wrote_an_autosynchronizer_for_f1viewer/

Great work @howlett