nanaimo_arguments should be settable from setup.cfg

[karlin]

The example in the documentation doesn't allow specifying argument values in setup.cfg:

With this in setup.cfg:

[nanaimo]
some_arg=foo

This test following the docs suggestion of usage fails:

def test_example(nanaimo_arguments: nanaimo.Namespace) -> None:
    assert nanaimo_arguments.some_arg is not None