Start making the archiving generic

go/archive/archive.go

	fmt.Println("==> Archiving", u)
	resultPath, err := archiveWithPrince(flags.archiveDir, u)
	var archiver string
	var archiveFunc func(string, *url.URL) (string, error)
	switch {
	default:
		archiver, archiveFunc = "prince", archiveWithPrince
	}
	resultPath, err := archiveFunc(flags.archiveDir, u)
	if err != nil {
		exit("prince", err)
		exit(archiver, err)
	}
	if archive.Mappings == nil {