feat: download single tracks (#5)

Introduces the `godeez download track <ID>` command, allowing users to download individual songs directly.

Single tracks are saved into a "Singles" folder by default to keep them organized. The README has been updated with documentation and examples for this new functionality.

cmd/download.go

@@ -35,6 +35,7 @@ func init() {
 		newDownloadCmd("album"),
 		newDownloadCmd("playlist"),
 		newDownloadCmd("artist"),
+		newDownloadCmd("track"),
 	)
 }
 
@@ -82,5 +83,9 @@ func newDownloadCmd(resourceType string) *cobra.Command {
 		cmd.Short = "Download top songs from an artist"
 	}
 
+	if resourceType == "track" {
+		cmd.Short = "Download a single track"
+	}
+
 	return cmd
 }