ataru can handle multiple files and test methods are named after md files

- Now, ataru can handle multiple files:

```bash
$ ataru my_file.md my_file_2.md
```

- Test method names contain the filename of the markdown file.
I think this case will never occur. The reason is that `ARGV` will always contains strings or will be empty. In the case of an empty array, the block will not be run at all. If you want to exit and print an error if no arguments where given at all you need to check if array is empty or not **before** `each` is called.