title: flake8: Ignore only F401 rule in entire file
body: <p>Is there a way to get <code>flake8</code> to ignore only a specific rule for an entire file? Specifically, I'd like to ignore just <code>F401</code> for an entire file.</p>
  
  <p>I have a file like <code>__init__.py</code> where I import symbols that are never used within that file. I'd rather not add <code># noqa</code> to each line. I can add <code># flake8: noqa</code> to the beginning of the file, but that ignores <strong>all</strong> rules. I'd like to ignore <strong>just</strong> the <code>F401</code> rule.</p>