Support basic SPARQL queries via Rulewerk

[mkroetzsch]

The VLog command line supports SPARQL (1.0) queries. It would be good to expose this feature in Rulewerk. This might first require an extension of the VLog API.

[mkroetzsch]

@CerielJacobs @jrbn Would it be possible to add this to the API? Rulewerk could pass queries as strings, but also could pre-process them in some way if useful.