added lstrip env var to jinja2

The lstrip_blocks var: (from http://jinja.pocoo.org/docs/api/#high-level-api)

> lstrip_blocks
>   If this is set to True leading spaces and tabs are stripped from the start of a line to a block. Defaults to False.

For example:

```
                                <ul class="taglist">
                                    <li><i class="icon-tags"></i></li>
                                    {% for tag in article.tags %}
                                    <li><a rel="tag" href="{{ SITEURL }}/{{ tag.url }}">{{ tag }}</a></li>
                                    {% endfor %}
                                </ul>
```

Generated:

```
                                <ul class="taglist">
                                    <li><i class="icon-tags"></i></li>
                                                                        <li><a rel="tag" href="/tag/pelican.html">pelican</a></li>
                                                                        <li><a rel="tag" href="/tag/blog.html">blog</a></li>
                                                                        <li><a rel="tag" href="/tag/rst.html">rst</a></li>
                                                                        <li><a rel="tag" href="/tag/python.html">python</a></li>
                                                                        <li><a rel="tag" href="/tag/html5.html">html5</a></li>
                                                                    </ul>
```

Now patched, generates:

```
                                <ul class="taglist">
                                    <li><i class="icon-tags"></i></li>
                                    <li><a rel="tag" href="/tag/pelican.html">pelican</a></li>
                                    <li><a rel="tag" href="/tag/blog.html">blog</a></li>
                                    <li><a rel="tag" href="/tag/rst.html">rst</a></li>
                                    <li><a rel="tag" href="/tag/python.html">python</a></li>
                                    <li><a rel="tag" href="/tag/html5.html">html5</a></li>
                                </ul>
```

:+1: