Added support for Features and Stories.

Added support for Features and Stories (https://github.com/allure-framework/allure-core/wiki/Features-and-Stories).
``severity(self, level`` explicitly states that one has to specify a severity level as a single argument.
``feature(self, *args)`` states that one may specify infinite number of ... arguments? 
At least change it to ``feature(self, *features)`` or ``feature(self, *feature_names)``