Querying a constructed graph with RDF filters.

Based on #246 we could also extend the current rdf filters to query a constructed graph object. But this is future work. (Please vote and comment if you require this feature.)

e.g.
```
{% assign customGraph = page.rdf | sparql_query: "construct {?s ex:prop "Is a Foo"} where {?s a ex:Bla }"%}
{{ page.rdf | rdf_property: "ex:prop", nil, nil, customGraph }}
```