Extend LDkit schema interface to allow for advanced operations

[karelklima]

For additional developer experience, LDkit should allow for the following:

• Matching of entities based on a partial entity, e.g. lens.match({ name: "Abc"});
• Working with huge arrays, e.g. lens.update({ $id, prop: { $add: "value"; });

This may require to extend LDkit language significantly. An alternative approach would be to use a proxy and translate actions on objects to SPARQL UPDATE queries, e.g. res.property.push("value") and then res.update(property) would yield the same result.

Consider using MongoDB language as an inspiration.

[karelklima]

Further enhanced by #80.