Allow SPARQL code in shape map's node selectors

[labra]

In order to validate RDF behind SPARQL endpoints, several ShEx implementation have extended node selectors in shape maps with SPARQL queries.

This allows, for example to use the following syntax:

SPARQL ' ' 'SELECT * WHERE {
 ?x rdfs:subClassOf*/rdf:type :Person .
?x :birthPlace :Spain 
}'''@<SpaniardShape>

to check all nodes that are instances of :Person and whose :birthPlace is :Spain to conform with <SpaniardShape>.

At this moment, both Shex.js and SHalclEX support this feature and with the recent adoption of ShEx in wikidata this feature is being used a lot.