Access nested content easily.

    k = NestedDict({'a': {'b': {'c': 123}}, 'd': {'e': 456}})

    assert k['a']['b']['c'] == 123
    assert k['b']['c'] == 123
    assert k['c'] == 123

    assert k[['a', 'b', 'c']] == 123
    assert k[['b', 'c']] == 123
    assert k[['c']] == 123

    assert k['a/b/c'] == 123
    assert k['b/c'] == 123
