title: make a title for <image> of SVG
body: <p>I was trying to put a title for a img of SVG.
  The title is at the bottom of the pic.
  The code goes like:</p>
  
  <pre><code>&lt;svg&gt;
    &lt;image&gt;
  &lt;/svg&gt;
  </code></pre>
  
  <p>How to realize this without using  inner ?
  And I also wanna make it based on <em>D3.js</em> , if possible. </p>
  
  <p>Thanks.</p>