title: Can e.g. pip list the options for extras_require?
body: <p>I know that I can specify optional dependencies in <code>setup.py</code> via <a href="http://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies" rel="noreferrer">extras_require</a>:</p>
  
  <pre><code>setup(
    name="Foo",
    extras_require={
      'bar':  ["barpy"],
    }
  )
  </code></pre>
  
  <p>Now, is there a way for a user to list the extras my package offers?  How else would a user know that it might be cool to install the package via</p>
  
  <pre><code>pip3 install .[bar]
  </code></pre>
  
  <p>?</p>