sources := cli.NewValueSourceChain()
app := &cli.Command{
		EnableShellCompletion:  true,
		UseShortOptionHandling: true,
		Before: func(ctx context.Context, cmd *cli.Command) (context.Context, error) {
			config := cmd.String("config")
			if len(config) > 0 {
				configFile := fmt.Sprintf("%s.yaml", cmd.String("config"))
				if _, err := os.Stat(configFile); os.IsNotExist(err) {
					return ctx, fmt.Errorf("could not load config file: %s", configFile)
				}

				sources.Append(cli.Files(configFile))
				return ctx, nil
			}

			return ctx, nil
		},
		OnUsageError: func(ctx context.Context, cmd *cli.Command, err error, isSubcommand bool) error {
			os.Exit(1)
			return err
		},
		ShellComplete: func(ctx context.Context, cmd *cli.Command) {
			fmt.Fprintf(cmd.Root().Writer, "check\nnotify\nvalidate\n")
		},
		Flags: []cli.Flag{
			&cli.StringFlag{
				Name:    "config",
				Aliases: []string{"c"},
				Usage:   "Load configuration from `FILE`",
			},
		},
		Commands: []*cli.Command{
			{
				Name:   "notify",
				Usage:  "Notify Telegram about updates",
				Flags:  []cli.Flag{
					&cli.StringFlag{
						Name:     "token",
						Required: true,
						Usage:    "`TOKEN` of Telegram's bot",
						//Sources:  altsrc.YAML("notify.telegram-chat", ???), // how to use sources/files from 'Before'?
					},
					&cli.StringFlag{
						Name:     "chat",
						Required: true,
						Usage:    "`ID` of Telegram's chat to send updates",
						//Sources:  altsrc.YAML("notify.telegram-chat", ???),  // how to use sources/files from 'Before'?
					},
				}
				Action: notify.Telegram,
			},