Implementation of options

This pull request allows syntactically for options on sections and variables:

* * * 

### Unless with "license" as first option
```html
<div class="entry">
  {{#unless license}}
  <h3 class="warning">WARNING: This entry does not have a license!</h3>
  {{/unless}}
</div>
```

### Log helper with two options
```html
{{log info "Look at me!"}}
```

```php
$engine= create(new MustacheEngine())->withHelpers(array(
  'log' => function($in, $context, $options) {
    Logger::getInstance()->getCategory('mustache')->debug($options);
    return '';  // Remove
  }
);
```

* * * 

Note this is *not* part of the mustache specification, but would allow easy creation by subclassing of [HandleBars](http://handlebarsjs.com/) support