title: how to set writeable highwatermark?
body: <p>I found in <code>options</code> to set the highwatermark for <a href="https://nodejs.org/dist/latest-v8.x/docs/api/fs.html#fs_fs_createreadstream_path_options" rel="nofollow noreferrer">readable stream</a>:
  <code>fs.createReadStream(path[, options])</code></p>
  
  <p>but cannot find <code>highwatermark options</code> for <a href="https://nodejs.org/dist/latest-v8.x/docs/api/fs.html#fs_fs_createwritestream_path_options" rel="nofollow noreferrer">writeable stream</a></p>
  
  <p>So how to set it when creating a writeable stream?</p>