Add support for toplevel arrays

[tonyxty]

Multiple issues including #248 call for jsonify to handle lists. As discussed there, allowing toplevel arrays should pose no security problem nowadays. A config parameter is added to enable this functionality, and an array is returned only when a conversion to an object is not possible to maintain backwards compatibility.

In short, when JSONIFY_ALLOW_TOPLEVEL_ARRAY is set, jsonify(1, 2, 3), jsonify(question='Ultimate', answer=42), and jsonify({'question': 'Ultimate', 'answer': 42}) all work as expected. Note that unlike dicts, jsonify([1, 2, 3]) will result in nested lists, so always use jsonify(*l) for a list variable l.