Allow grouping of stories and scenarios

Introduce the concept of group of stories or scenarios, which can be dynamically maintained based on some criteria.

A group should be identified by a new keyword but could reference stories and scenarios anywhere in the run context, i.e. not necessarily in the same textual input.

A group should use story and scenario meta data, using pre-defined tags or user-defined ones, to determine its content.

A group should be used as an execution context to allow repetition of stories or scenarios with some overall directives and fail/success criteria.