Format:
        
        {% alert class=error %}
        message
        {% endalert %}

def new_bootstrap_alert(visitor, block):
    """
    Format:
        
        {% alert class=error %}
        message
        {% endalert %}
    """
    if 'new' in block:
        txt = []
        cls = block['kwargs'].get('class', '')
        if cls:
            cls = 'alert-%s' % cls
        txt.append('<div class="alert %s">' % cls)
        if 'close' in block['kwargs']:
            txt.append('<button class="close" data-dismiss="alert">&times;</button>')
        text = visitor.parse_text(block['body'], 'article')
        txt.append(text)
        txt.append('</div>')
        return '\n'.join(txt)
    else:
        return bootstrap_alert(visitor, block)