cp (from, to) {
    return new Promise((resolve, reject) => {
      if (!this.enableSsh) {
        return reject(new Error('do not support copy operation in sftp only mode'))
      }
      const { client } = this
      const cmd = `cp -r "${from}" "${to}"`
      client.exec(cmd, this.getExecOpts(), (err) => {
        if (err) reject(err)
        else resolve(1)
      })
    })
  }
cp

@param {String} from
@param {String} to
https://github.com/mscdex/ssh2/blob/master/SFTP.md
@return {Promise}