pr-read-in-memory

I dig around "archive/zip", then found a way to read a zip file in memory.But it need some change to `github.com/tealeg/xlsx`
caller code
```golang
	r := bytes.NewReader(buf.Bytes())
	zr, err := zip.NewReader(r, int64(buf.Len()))
	t.Equal(err, nil)
	xlsxFile, err := xlsx.ReadZipReader(zr)
	t.Equal(err, nil)
```
https://github.com/bronze1man/kmg/blob/master/encoding/kmgExcel/WriteRead_test.go#L12