Support for importing raw templates added

This PR relates to discussion in https://github.com/fabpot/Twig/issues/1229

Usage
```
{% include_raw "my_sweet_template.html.twig" %}
```

Disclaims
- I just learned Twig internals today, so implementation may be done wrongly even if it seems to work
- I had to add a new method to Environment.php
- No BC breaks are introduced.
- Tests are not (yet) implemented because I'd like to hear first if this has any future.