code: function updateExample(app, str) {
    var cwd = app.options.dest || app.cwd;
    return app.src('example.txt', {cwd: cwd})
      .pipe(append(str))
      .pipe(app.dest(cwd));
  }
docs: Append the given string to `example.txt` and re-write the file to the current
  working directory.
  
  The `.src` and `.dest` methods work exactly like gulp's (we use the same libs from the
  gulp team under the hood)
  
  @param {Object} `app` Instance of update, to get the cwd. Pass `--dest` on the command line to set `app.options.dest`
  @return {Stream} vinyl stream
  @api public