title: pytest-html add custom test results to report
body: <p>I'm using pytest0html to generate my html report. my test record tested values and I need to show a table of this values with prettytable in case of success. 
  I think that i need to implement this hook:</p>
  
  <pre><code>@pytest.mark.optionalhook
  def pytest_html_results_table_html(report, data):
      if report.passed:
          del data[:]
          data.append(my_pretty_table_string)
          # or data.append(report.extra.text)
  </code></pre>
  
  <p>But how to pass my_pretty_table_string to the hook function or how to edit report.extra.text from my test function ?
  Thank you for your help</p>