Allow adding custom flags to the ffmpeg process

[laggingreflex]

Please allow passing custom flags to the ffmpeg process.

I wanted to add an audio by passing it as an input to the ffmpeg process.

Also, have you considered fluent-ffmpeg? Maybe using and exposing it would be better than exposing the arguments array that you currently pass to spawn

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Ah, sorry, I'm too lazy to read. It seems this also already exists.