Added support for optional raw arguments

This pull request does not break any use case with the raw tag instead it will parse a given argument when given as boolean and enable raw depending on that argument.

For example:

```
{% spaceless %}
<section>
{% raw %}<p>I like these brackets {{ }} yeah{% endraw %}
{% raw client %}
<h3>{{ name }}</h3>
<!-- can be used by client -->
{% endraw %}
</section>
{% include './somefile.html' %}
{% endspaceless %}
```

Please review if everything is okay so far. Also you may have an idea how to remove code duplication on parser implementation of raw (is copied from the if tag).