code: function stream(out, err) {
    if(!err) err = out;
  
    if(typeof out === 'string') {
      this._out = fs.createWriteStream(out);
    }else{
      this._out = out;
    }
  
    if(typeof err === 'string') {
      this._err = fs.createWriteStream(err);
    }else{
      this._err = err;
    }
  
    if((!(this._out instanceof Writable)
        && this._out !== process.stdout)
      || (!(this._err instanceof Writable)
        && this._err !== process.stderr)) {
      throw new Error('invalid log stream');
    }
  }
docs: Set the output stream(s).