Made story title prefix customisable.

Implements [#68](https://github.com/TestStack/TestStack.BDDfy/issues/68). Allows you to customise the story title prefix for all reports by setting TitlePrefix property in the Story attribute. Defaults to current behaviour of "Story: "
Implements [#95](https://github.com/TestStack/TestStack.BDDfy/issues/95). Added RunDate to FileReportModel and removed date funcs.