Added support for Features and Stories.

Added support for Features and Stories (https://github.com/allure-framework/allure-core/wiki/Features-and-Stories).
Description of severity method differs only by one word.

```
    def severity(self, level):
        """
        A decorator factory that returns ``pytest.mark`` for a given allure ``level``.
        """
        return pytest.mark.allure_severity(level)
```
 Could you please explain your comment.