Added macro and import support to twigjs

Thanks for very detailed explanation in #21. I tried to keep it identical to Twig.

```
{% macro func(params) %}{% endmacro %}
{% import "macro.twig" as variable %}
{% import _self as variable %}
{% from "macro.twig" import func as blah, echo %}
```

Hoping to get it merge soon, thanks for great work :+1: