Optionally validate output with ruby-rdf

[wstrinz]

Currently the gem does not check whether its output is syntactically valid turtle rdf. Ruby-rdf's Repository class normally just ignores invalid statements, but it can be set to raise an error on invalid input by passing the validate: true parameter to an RDF::Repository.load() call.

The gem DSLs to_repository method (see here) uses the RDF::Repository interface, so adding the option to validate output should be as simple as adding the validate: true flag to the repo.load call, and handling and informing users about potential errors.