Added macro and import support to twigjs

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