Allow for including options from parent `argparse.ArgumentParser`
objects in preparation of splitting out the handling of preliminary
options from the `OptionManager`.

When calling `ArgumentParser.parse_args()` with the `namespace`
argument, command-line options are just added to the  namespace without
going through any of the argument parsing and type conversion logic
(e.g., the `type` keyword argument of `ArgumentParser.add_argument()`).
In other words, it is assumed that a namespace is well-formed from a
previous invocation of `ArgumentParser.parse_args()`.

The `values` parameter is intended to be values already-provided from
configuration files.  To take advantage of the logic defined by
`ArgumentParser.add_argument()`,  utilize
`ArgumentParser.set_defaults()` instead.

Ensure options provided external to the command-line (i.e.,
configuration files) are present in the final result of options.