Add Narrative elements to Story parsing

The current domain model captures everything before the first Scenario as unstructured text (hence the name Blurb in the domain module). Hence not an issue in a plain text report (newlines are maintained) but becomes less readable in, say, HTML.

We should add an optional Narrative element with appropriate keywords, so as to be able to distinguish between the various elements of the narrative ("In order to", "As a", "I want to").