Added support for Features and Stories.

Added support for Features and Stories (https://github.com/allure-framework/allure-core/wiki/Features-and-Stories).
Why not just ``Label``? Labelling itself has nothing to do with testing -- it is a separate concept.
We dont have ``TestAttach`` or something -- in fact, ``Test`` prefix should only be used in the objects that can be treated as some general ``test`` -- i.e. the ones that may pass or fail. Label can not fail, therefore it should not have the ``Test`` prefix.