Fix issue #13

```
$ python modernize.py --help
Python           _              _
   _ __  ___  __| |___ _ _ _ _ (_)______
  | '  \/ _ \/ _` / -_) '_| ' \| |_ / -_)
  |_|_|_\___/\__,_\___|_| |_||_|_/__\___| 0.3

Usage: modernize [options] file|dir ...

Options:
  --version             show program's version number and exit
  -h, --help            show this help message and exit
  -d, --doctests_only   Fix up doctests only
  ...
```