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.

internal/downloader/client.go

@@ -63,6 +63,8 @@ func (c *Client) Run(ctx context.Context, opts Options, id string) error {
 		resource = &deezer.Playlist{}
 	case "artist":
 		resource = &deezer.Artist{}
+	case "track":
+		resource = &deezer.Track{}
 	default:
 		return fmt.Errorf("unsupported resource type: %s", c.resourceType)
 	}