Load the Jinja2 'do' expression-statement extension.

This allows manipulation of Jinja2 data structures without setting a variable value, e.g. to add a pair to a dict:

```
{% do MYDICT.update( { KEY : VALUE } ) %}
```

Without the extension you have to do this:

{% set JUNK = MYDICT.update( { KEY : VALUE } ) %}

@matthewrmshin - please review.