title: Heroku: stop all one-off dynos without dyno name
body: <p>I run the following command twice to start my two one-off dynos</p>
  <pre><code>heroku run:detached python main.py
  </code></pre>
  <p>and then stop it using</p>
  <pre><code>heroku ps:stop &lt;dyno-name&gt;
  </code></pre>
  <p>however, I wish to stop all my dynos without knowing dyno names before hand, since I am trying to automate turning on and off my dynos using crontab.</p>