Support io.Reader interface

[kodawah]

it would be nice if NewReader() returned an io.Reader interface, so that the reader could be chained across other readers (for example you could open a file or a http body request, wrap a decompressor, and return a generic reader.

for example

func loadData(pathOpt string) (io.ReadCloser, error) {
	var reader io.ReadCloser

	u, err := url.Parse(pathOpt)
	if err != nil {
		return nil, err
	}
	switch u.Scheme {
	case "http", "https":
		resp, err := cleanhttp.DefaultClient().Get(pathOpt)
		if err != nil {
			return nil, err
		}

		reader = resp.Body
	case "b2":
		src := strings.TrimPrefix(u.Path, "/")
		obj := B2Bucket.Object(src).NewReader(context.TODO())
		obj.ConcurrentDownloads = 20

		reader = obj
	default:
		file, err := os.Open(pathOpt)
		if err != nil {
			return nil, err
		}

		reader = file
	}

	if strings.HasSuffix(pathOpt, "xz") {
		xzReader, err := xzReader.NewReader(reader, 0)
		if err != nil {
			return nil, err
		}
		reader = io.NopCloser(xzReader)
	} else if strings.HasSuffix(pathOpt, "bz2") {
		bz2Reader, err := bzip2.NewReader(reader, nil)
		if err != nil {
			return nil, err
		}
		reader = bz2Reader
	} else if strings.HasSuffix(pathOpt, "zip") {
		zipReader := zipstream.NewReader(reader)
		reader = io.NopCloser(zipReader) // <---- can't do this
	}

	return reader, err
}