Added support for Features and Stories.

Added support for Features and Stories (https://github.com/allure-framework/allure-core/wiki/Features-and-Stories).
I'm not quite sure, but it appears to me that this may return not the result you expect it to. For example, if there is a label with no ``value`` attribute, the ``zip`` will get truncated to a one less label. You'd better look up all the ``label`` nodes and iterate over them with a list comprehension.

However, this method should be completely reworked thanks to the comment at line 38.