Allow passing ffmpeg options

[TatriX]

Hi! The tool works great, thank you!

I wanted to change default audio stream so I had to patch source code and rebuild:

func (a *Application) serveLiveStreaming(w http.ResponseWriter, r *http.Request, filename string) {
	cmd := exec.Command(
		"ffmpeg",
		"-re", // encode at 1x playback speed, to not burn the CPU
		"-i", filename,
		"-vcodec", "h264",
		"-acodec", "aac",
		"-ac", "2", // chromecasts don't support more than two audio channels
/**/		"-map", "0:v", // Select video
/**/		"-map", "0:2", // Select second stream with specific audio track
		"-f", "mp4",
		"-movflags", "frag_keyframe+faststart",
		"-strict", "-experimental",
		"pipe:1",
	)

It would be super cool if one could pass something like --audio-track "title" to select a specific audio track.
Otherwise ability to pass custom ffmpeg flags would solve that as well. For example castnow allows for --ffmpeg-<option> <arg>.

Thank you!

[vishen]

This would be quite a cool and useful feature!