title: List all environment id in openai gym
body: <p>How to list all <strong>currently registered</strong> environment IDs (as they are used for creating environments) in openai gym? </p>
  
  <p>A bit context: there are many plugins installed which have customary ids such as atari, super mario, doom etc.</p>
  
  <p>Not to be confused with game names for atari-py.</p>