title: Add a dtd using nokogiri builder
body: <p>I am using <code>nokogiri</code> to generate svg pictures. I would like to add the correct xml preamble and svg DTD declaration to get something like:</p>
  
  <pre><code>&lt;?xml version="1.0" standalone="no"?&gt;
  &lt;!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd"&gt; 
  &lt;svg&gt;
  ...
  </code></pre>
  
  <p>With <code>builder</code> I could use <code>instruct!</code> and <code>declare!</code> but I want to stick with <code>nokogiri</code> because I use it for other purpose in my project and I want to stay low on requirements. Do you have some ideas ?</p>
  
  <p>Thanks</p>