Added support for Features and Stories.

Added support for Features and Stories (https://github.com/allure-framework/allure-core/wiki/Features-and-Stories).
Technically, that's not multiple markers -- that's one marker with several features specified.
Multiple markers would be:
```python
@allure.feature('A')
@allure.feature('B')
def test_foo():
    pass
```

By the way, does this work?