Hoist passing through sys.argv at the CLI layer

`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]`.