Generator: Add preamble method.

[Insti]

Add a method to the ExerciseCases class that can be overridden to provide common setup code when using the default template.

Ref:
https://github.com/exercism/xruby/pull/583/files#r114226935 <- This should not require a custom template.

[hilary]

I believe there's at least one more exercise (bowling, I think) that would benefit from such a method.

My guiding principle on these has been to wait until I have more than one use for such a method before adding it to ExerciseCases.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.