ENH: Add pyld CLI console_script

[westurner]

This is lacking actual tests from http://json-ld.org/test-suite/ "Transform RDF to JSON-LD tests", but does seem to work for the simple --rdf-to-jsonld case.

I may have been a bit overeager in terms of adding commandline arguments for as-yet-untested CLI functionality (which is already appropriately tested at a lower-level, according to http://json-ld.org/test-suite/reports/ ).

Questions:

• Would this be a useful addition to PyLD?
• How should the tests be structured, in relation to the existing tests?
• I've been testing with http://topbraid.org/schema/schema.ttl , which does take about a minute per test on this machine. (http://topbraid.org/schema/)
• Eventually, I'd like to generate a proper @context file, rather than a direct transformation from RDF/TTL -> Quads -> JSON-LD.
• Could there be an RDFLib document loader, with support for all of the formats which RDFLib supports?
• DOC: How do I clone and run the existing tests?