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.
`Pathname` is a great class. Very neat you used it here. You perform some checks on the `file_name` in your executable, too. It would be possible to use a `Pathname`-instance there too and then pass it to this function. The downside would be, that you could not pass simple strings to `add_test_cases`. But it is something one could think  about :-)