Added embed support

Added `embed` support which was introduced in Twig 1.8
A bit difficult to read this test's expected output.
I think you could modify your test twig templates this way:
```twig
{% block header %}
base header
{% endblock %}
```
while the the expected output could look like this:
```javascript
[
  'Start',
  'A',
  'new header',
  'base footer'
].join('\n')
```
I think, it could radically improve readability of the test case.