ffmpeg fix merge (#85)

Author: daktak

pkg/videomanipulation/ffmpeg.go

@@ -117,7 +117,7 @@ func (v *VMan) merge(output string, bar *mpb.Bar, ffConfig FFConfig, videos ...s
 
 func (v *VMan) Merge(bar *mpb.Bar, videos ...string) error {
 	mergeConfig := v.NewDefaultConfig()
-	mergeConfig.InArgs = append(mergeConfig.InArgs, []string{"-f", "concat", "-safe", "0"}...)
+	mergeConfig.InArgs = append(mergeConfig.InArgs, []string{"-f", "concat", "-safe", "0", "-ignore_unknown"}...)
 	mergeConfig.OutArgs = append(mergeConfig.OutArgs, []string{"-map", "0:0", "-map", "0:1", "-map", "0:3"}...)
 
 	err := v.merge(getMergedOutputFilename(videos[0]), bar, mergeConfig, videos...)