Dedent html examples

Usually styleguide example nested into others blocks and has an additional
indent level

```jinja
{% block main %}
  <h3>Buttons</h3>

  {% styleguide 42 %}
    <button class="button $modifier_class">
      <i></i>A button
    </button>
  {% endstyleguide %}
{% endblock %}
```

That produce an ugly result when leading whitespaces injected into markup

```html
<pre>
    <button class="button $modifier_class">
      <i></i>A button
    </button>
</pre>
```

instead of

```html
<pre>
<button class="button $modifier_class">
  <i></i>A button
</button>
</pre>
```