ENH: Added new JSONformatter to write Ostream content in JSON format

New Ostream wrapper that writes entries as JSON objects/convert OpenFOAM
dictionaries to JSON dictionaries, e.g.

    dictionary dict ...

    // Create file for writing
    OFstream os("myData.json");

    // Wrap Ostream
    JSONformatter json(os);

    // Write dictionary as JSON
    json.writeDict(dict);