title: Put a button inside the <a></a> tag using pug.js
body: <p>this is my expected result:</p>
  
  <pre><code>&lt;a&gt;
      &lt;button class="btn btn-tomato mx-auto" type="button" style="width:50%; white-space: normal; opacity:0.8;"&gt;Button text&lt;/button&gt;  
  &lt;/a&gt;
  </code></pre>
  
  <p>So I wrote my pug like this:</p>
  
  <pre><code>a
      button.btn.btn-tomato.mx-auto(type='button', style='width:50%; white-space: normal; opacity:0.8')
  </code></pre>
  
  <p>However, the result is like this:</p>
  
  <pre><code>&lt;a&gt;&lt;/a&gt;
  &lt;button class="btn btn-tomato mx-auto" type="button" style="width:50%; white-space: normal; opacity:0.8;"&gt;Button text&lt;/button&gt;
  </code></pre>
  
  <p>The <code>&lt;button&gt;</code> is not in the <code>&lt;a&gt;</code>. Would you please tell me how to fix that? thanks.</p>