bin: Exit with the number of "suggestions"

This patch makes the binary (`write-good(1)`) a bit more "unixy".  The
prior implementation exited with `0` unless a runtime error occurred.

Now running this:

```sh
$ bin/write-good.js README.md > /dev/null && echo "hi"
```

Should not output "hi".
Merge pull request #30 from stephenmathieson/fix/exit-code

bin: Exit with the number of "suggestions"