add -r option, it will read all from http response body

[shaneing]

When the response body is a stream, there will be a big difference. For example, download a file.

In order to avoid OOM, we can use ioutil.Discard. For example:

if *readAll {
  n, err = io.Copy(ioutil.Discard, resp.Body)
  respObj.Size = n
  return
}