Recognize variables in class parameters

This might fix https://github.com/eklein/sublime-text-puppet/issues/4 (but this is my first time looking at the tmLanguage syntax definition, so I could be completely wrong).

Previously, having this class parameter:

```puppet
class some_class(
  $class_param = "puppet:///modules/foo/bar/${::hostname}.py"
) inherits some_other_class {
    # class definition
}
```

broke syntax highlighting with all text past the `.py"` being considered part of a string. After this change, this is no longer the case and all my `.pp` files look correctly syntax-highlighted.