Allow and apply Trait variables in RAML

The Parser throws warnings when the trait is included with variable parameters.

Example from [here](http://git1.meteoplus.de/meteoplus-api/spec/raw/master/raml/meteoplus-api.raml) (Expandable node will fail)
```
          is:
            - LocationBased
            - Expandable: {example: "minutes", on: "minutes, location"}
```

Fixed and also implemented a basic variable replacement function.