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.
`load_file` is defined "globally" in the `markdown_loader`. Adding methods "globally" is very dangerous: Imagine there is another `load_file` method defined by someone else. You would get some strange errors and lose some serenity in the end. Just wrap this method into a class (like the `add_test_cases` method on `CodeSamples`).