Fix bin/kpeg when there are no arguments passed in

Was just playing around with a simple parser and noticed the following happens when you don't pass any arguments to kpeg:

```
sjothen@air% kpeg                                                    
/Library/Ruby/Gems/1.8/gems/kpeg-0.10.0/bin/kpeg:49:in `exists?': can't convert nil into String (TypeError)
	from /Library/Ruby/Gems/1.8/gems/kpeg-0.10.0/bin/kpeg:49
	from /usr/bin/kpeg:19:in `load'
	from /usr/bin/kpeg:19
```

This PR just adds a check after option parsing to print out the help information if no arguments are passed in.