Improves castInput to allow any JSON object as attribute value

[usr-ein]

Following PR #15 , this introduces a better way to parse attribute values provided by the user. Does so by leveraging json.loads() ability to identify and convert into Python equivalent any JSON object, including complex values like lists, objects etc. Doesn't break current single atomic values conversion, i.e. type(json.loads("1")) == int, type(json.loads("true")) == bool, etc.

Before the suggested changes:

Current value of my_attr is: {'attr1': 'foo', 'attr2': 'bar'}
Please enter new value and hit ENTER
> {"attr1": "food", "attr2": "baz"}
[1] my_attr = "{'attr1': 'food', 'attr2': 'baz'}"   # This is a string and not an object !

And after

Current value of my_attr is: {'attr1': 'foo', 'attr2': 'bar'}
Please enter new value and hit ENTER
> {"attr1": "food", "attr2": "baz"}
[1] my_attr = {'attr1': 'food', 'attr2': 'baz'}  # This is a a JSON object, i.e. a Python dict