Feature request: show_rubric option

Giving students the grading rubric in some form along with the exercise has multiple benefits:

- clearer and more explicit requirements for learners
- incentives instructors to write higher quality and more objective rubrics

For some exercises it may not make sense -- it may not always be possible to write a rubric without "giving away" part of the exercise. But it'd be nice to be able to show it for some/most exercises.

For coding exercises without a "rubric" option it would probably make more sense to describe the test code behavior in the prompt instead, so I'm thinking it would only apply to flows with a "rubric" option.