Fix ignore_blocks with non-zero indentation

[tdryer]

The ignore_blocks option skips indentation of content inside ignored template block tags. This works by skipping indentation when a block begins. However, it does not skip un-indentation when a block ends, causing unexpected results when a block has a non-zero indentation level.

Expected:

<p>
    {% raw %}
    test
    {% endraw %}
</p>

Actual:

<p>
    {% raw %}
    test
{% endraw %}
</p>

Fix this by modifying the template_unindent pattern to exclude tags that are in ignore_blocks via a negative lookahead.