feature: Group By a Property Name

I'd love to use GroupBy a Property. 

```
groupBy: 'Your.Package:GroupType' # If it's just a parent nodetype
groupByPropertyName: 'category'   # In addition to `groupBy`, if you'd like to group by a reference field
```

If only groupByPropertyName is set, it will group the Nodes only by Property maybe?

as discussed @jobee 