ENH: Add pyld CLI console_script

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?