title: AsciiDoctor: How can I add custom xmlns'
body: <p>How can I add a custom <code>xmlns</code> in the output when I convert an asciidoc file with AsciiDoctor?</p>
  
  <p>I'd like to add <code>xmlns:xi="http://www.w3.org/2001/XInclude"</code> in the top  <code>book</code> tag.</p>
  
  <p>The current implementation seems to generate:</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;?asciidoc-toc?&gt;
  &lt;?asciidoc-numbered?&gt;
  &lt;book xmlns="http://docbook.org/ns/docbook" xmlns:xl="http://www.w3.org/1999/xlink" version="5.0" xml:lang="en"&gt;
  &lt;info&gt;
  &lt;title&gt;title&lt;/title&gt;
  &lt;/info&gt;
  &lt;/book&gt;
  </code></pre>
  
  <p>from this:</p>
  
  <pre><code>= title
  :lang: en
  </code></pre>
  
  <p>When I run:</p>
  
  <pre class="lang-sh prettyprint-override"><code>$ asciidoctor -b docbook5 -d book -o out.xml source.txt
  </code></pre>
  
  <p>There is a <a href="http://asciidoctor.org/docs/user-manual/#builtin-attributes" rel="nofollow noreferrer">built-in attribute <code>xmlns</code></a>, but it seems to be for docbook 4.5.</p>
  
  <p>The reason I want to use XInclude is to include some xml files from <a href="http://asciidoctor.org/docs/user-manual/#docinfo-file" rel="nofollow noreferrer">Docinfo files</a> and <a href="http://asciidoctor.org/docs/user-manual/#pass-blocks" rel="nofollow noreferrer">Passthrough Blocks</a></p>