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.

Additionally, the contract for various argument parsing methods to be
guaranteed a `List[str]`.