Add JSON export format.

[yohcop]

JSON format does not download any file, but saves all the metadata received about activities in a .json file.

This is useful to load all the metadata for analysis using other tools. CSV is good too, but by exporting the raw data as JSON, everything is available and parsing is easy (for example, normalized numbers without units are available).

[yohcop]

FWIW, I'm happy rewriting the change after the argparse lib change.