Add --audio-only flag (#1304)

* Add --audio-only flag

* Resolve all conflicts

* Fix go lint errors

app/app.go

@@ -89,6 +89,11 @@ func New() *cli.App {
 				Aliases: []string{"f"},
 				Usage:   "Select specific stream to download",
 			},
+			&cli.BoolFlag{
+				Name:    "audio-only",
+				Aliases: []string{"ao"},
+				Usage:   "Download audio only at best quality",
+			},
 			&cli.StringFlag{
 				Name:    "file",
 				Aliases: []string{"F"},
@@ -300,6 +305,7 @@ func download(c *cli.Context, videoURL string) error {
 		Silent:         c.Bool("silent"),
 		InfoOnly:       c.Bool("info"),
 		Stream:         c.String("stream-format"),
+		AudioOnly:      c.Bool("audio-only"),
 		Refer:          c.String("refer"),
 		OutputPath:     c.String("output-path"),
 		OutputName:     c.String("output-name"),

downloader/downloader.go

@@ -28,6 +28,7 @@ type Options struct {
 	InfoOnly       bool
 	Silent         bool
 	Stream         string
+	AudioOnly      bool
 	Refer          string
 	OutputPath     string
 	OutputName     string
@@ -568,6 +569,26 @@ func (downloader *Downloader) Download(data *extractors.Data) error {
 		return errors.Errorf("no stream named %s", streamName)
 	}
 
+	if downloader.option.AudioOnly {
+		var isFound bool
+		reg, err := regexp.Compile("audio+")
+		if err != nil {
+			return err
+		}
+
+		for _, s := range sortedStreams {
+			// Looking for the best quality
+			if reg.MatchString(s.Quality) {
+				isFound = true
+				stream = data.Streams[s.ID]
+				break
+			}
+		}
+		if !isFound {
+			return errors.Errorf("No audio stream found")
+		}
+	}
+
 	if !downloader.option.Silent {
 		printStreamInfo(data, stream)
 	}