Make output filterable using libjq

[jplitza]

There is library version of the well-known JSON processor jq, and there are python bindings for it. It would be great to specify a jq filter on the command line or in a file which is applied to the/every output JSON file or alternatively to the JSON input from the nodes.

This would solve #78, #44 and all other problems where some kind of modification is required. Those are trickling in one by one and are each asking for a separate option which will blow up the code for some rare use cases.