Add console script 'mccabe' for usability

Hi!
Thanks for useful tool! I use it at [git-pre-commit-hook](https://pypi.python.org/pypi/git-pre-commit-hook)

For debug reasons I often run mccabe alone without flake8.
For me run
```
mccabe file.py
```
is easier than
```
python -m mccabe file.py
```

Yes, bash alias can help. But every developer must define that alias.