title: Using a Jade mixin block as an attribute
body: <p>Whilst developing a library of Jade templates it has become desirable to use a mixin's block to serve as an attribute value, simplifying the syntax for the end user.</p>
  
  <p>The end user will have a choice of 3 ways to create buttons; via a tags, button tags and input tags. For the input tags I would like to use the block as the value attribute so the syntax is always:</p>
  
  <pre><code>+abtn
      | A Button
  +btn
      | Button
  +ibtn
      | I Button
  +abtn(disabled)
      | A Button Disabled
  +btn(disabled)
      | Button Disabled
  +ibtn(disabled)
      | I Button Disabled
  </code></pre>
  
  <p>At present a slimmed down version of the mixins looks like:</p>
  
  <pre><code>mixin abtn
      - attributes.href = attributes.href || '#'
      - attributes.role = attributes.role || 'button'
      - if (attributes.disabled) {
      -     attributes.class = (attributes.class === undefined) ? 'disabled' : attributes.class + ' disabled';
      -     attributes.disabled = null
      - }
      a.btn(attributes)
          block
  
  mixin btn
      - attributes.type = attributes.type || 'button'
      button.btn(attributes)
          block
  
  mixin ibtn
      - attributes.class = (attributes.class === undefined) ? 'btn' : attributes.class + ' btn';
      - attributes.type = attributes.type || 'button'
      input(attributes=attributes)
  </code></pre>
  
  <p>The trouble is specifying the value attribute for ibtn requires the end user syntax to be:</p>
  
  <pre><code>+abtn
      | A Button
  +btn
      | Button
  +ibtn(value='I Button')
  +abtn(disabled)
      | A Button Disabled
  +btn(disabled)
      | Button Disabled
  +ibtn(value='I Button Disabled', disabled)
  </code></pre>
  
  <p>Which is inconsistent.</p>
  
  <p>Is it possible to access block via the inbuilt javascript, so that a non-whitespace version of its contents can be used in an attribute? If so how?</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>To clarify, I would like the following jade code:</p>
  
  <pre><code>+ibtn
        | My button value
  </code></pre>
  
  <p>to output:</p>
  
  <pre><code>&lt;input value="My button value"&gt;
  </code></pre>