title: reStructuredText substitution in image options/path
body: <p>Is there an option to substitute image options/path in reStructuredText?</p>
  <p>Something like:</p>
  <pre><code>.. image:: |picdir|/someImage.jpg
     :alt: SomeImage
     :width: |imagewidth|
     :target: |picdir|/someImage.jpg
  
  
  .. |picdir| replace:: ./pic
  .. |imagewidth| replace:: 500px
  </code></pre>
  <p>I found similar <a href="https://stackoverflow.com/questions/20513972/substitution-in-a-file-name-with-restructuredtext-sphinx">question</a> but it is quite old and I am hoping that a solution was developed now.</p>