Option to serialise csvw to file

[Robsteranium]

With #27 we're now passing the json metadata to csv2rdf with an in-memory map (rather than writing it out to disk). This is designed to make the whole process more memory efficient when called via grafter-server/ pmd.

Ideally we would also support the original use case - so that people could use table2qb as a standalone, writing the csvw output (i.e json and csv files). This would allow users to pick a different csv2rdf implementation, or to target csvw itself.

[Robsteranium]

Not being able to do this means the examples need to be updated by hand - I'm only worrying about those used in the tests for now.

Once the above is solved we ought to think about adding an "update examples" task or script.

[Robsteranium]

This is closed by #120, thanks @lkitching.

I've opened PR #123 extending on this to add a script that updates the examples.