title: how to modify config files using gulp
body: <p>I use gulp to configure complex local setup and need to auto-edit files.</p>
  
  <p>The scenario is:</p>
  
  <ul>
  <li>determine if certain file contains certain lines after certain other line (found using regular expression)</li>
  <li>if line is not found, insert the line. </li>
  <li>optionally, delete some lines found in the file.</li>
  </ul>
  
  <p>I need this to amend system configuration files and compile scenarios.</p>
  
  <p>What would be the best way to do it in gulp? </p>