Implement archiving for youtube and vimeo

Untested, but could work.  Would be nicer with previews and stuff, but
that's optional and for later.  (Or even better: using different tools.)

go/archive/archive.go

	var archiveFunc func(string, *url.URL) (string, error)
	switch {
	case u.Host == "youtube.com" || u.Host == "www.youtube.com" || u.Host == "youtu.be" ||
		u.Host == "vimeo.com":
		archiver, archiveFunc = "youtube-dl", archiveWithYoutubeDL
	default:
		archiver, archiveFunc = "prince", archiveWithPrince
	}
	}
}
func archiveWithYoutubeDL(dir string, u *url.URL) (resultPath string, err error) {
	cmd := exec.Command("youtube-dl", "--get-filename", u.String(), "--output", "%(url)s.%(ext)s")
	cmd.Dir = dir
	cmd.Stderr = prefixWriter("    | ", os.Stderr)
	buf := new(bytes.Buffer)
	cmd.Stdout = buf
	fmt.Print("    ", cmd.Args[0])
	for _, arg := range cmd.Args[1:] {
		fmt.Print(" ", arg)
	}
	fmt.Println()
	err = cmd.Run()
	if err != nil {
		return "", err
	}
	p := path.Join(dir, buf.String())
	cmd = exec.Command("youtube-dl", u.String(), "--output", "%(url)s.%(ext)s")
	cmd.Dir = dir
	cmd.Stderr = prefixWriter("    | ", os.Stderr)
	cmd.Stdout = prefixWriter("    | ", os.Stdout)
	fmt.Print("    ", cmd.Args[0])
	for _, arg := range cmd.Args[1:] {
		fmt.Print(" ", arg)
	}
	fmt.Println()
	err = cmd.Run()
	if err != nil {
		return "", err
	}
	return p, nil
}
func archiveWithPrince(dir string, u *url.URL) (resultPath string, err error) {
	buf := make([]byte, 16)
	_, err = rand.Read(buf)