add fetch-one command

go/feeds/feeds.go

	switch cmd {
	case "fetch-all":
		FetchAll()
	case "fetch-one":
		if len(flag.Args()) != 1 {
			fmt.Printf("Usage: %s [<options>] fetch-one <url>\n", os.Args[0])
			os.Exit(1)
		}
		FetchOne(flag.Args()[0])
	case "help":
		printUsage()
		flag.PrintDefaults()
	fmt.Printf("Usage: %s [<options>] <cmd> [<args>]\n", os.Args[0])
}
func FetchOne(u string) {
	f, err := Fetch(u)
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
	fmt.Printf("%s: %s", u, f.Title)
	if strings.TrimSpace(f.Description) != "" {
		fmt.Printf(" - %s", f.Description)
	}
	fmt.Printf(" (%d entries)\n", len(f.Items))
	for _, item := range f.Items {
		fmt.Printf("\t%s\n", item.Title)
	}
}
func FetchAll() {
	feeds, err := ReadConfig("config.txt")
	if err != nil {