Leverage the `type` keyword argument to `ArgumentParser.add_option()` for
converting the raw string, either from a configuration file or the
command-line, into a data structure for later use.
Now with `OptionManager.parse_args()` feeding configuration file values
as default values to be considered by the `ArgumentParser.parse_args()`
framework, normalization of the file paths occur in single place.
`flake8.main.cli.main()` is the primary entry point for the command-line
implementation of flake8 (invoked via `__main__` or `console_scripts`).
Therefore, it is reasonable for the entry point to be responsible for
obtaining command line arguments from `sys.argv` there.
Note that `sys.argv[1:]` is necessary in order to strip off the script
name. Formerly, this was not needed in
`Application.parse_preliminary_options_and_args()`, which was using
`sys.argv[:]` because the result of the argument parsing was just for
determining additional configuration to be loaded. Then, the *real* CLI
argument parsing was forwarding the original `None` value to
`argparse.ArgumentParser.parse_args()`, which internally was obtaining
arguments as `sys.argv[1:]`.
Additionally, the contract for various argument parsing methods to be
guaranteed a `List[str]`.
