add (simple) function to write student-friendly version of grading rubric

[federicazoe]

write_friendly_rubric(rubric_path, friendly_format = TRUE)

This could be a function that writes a simplified version of the rubric used for grading, with three columns: question name, feedback, score. When friendly_format = TRUE, some make up is done to question names to make them more readable, e.g. underscores/hyphens in question names are substituted with spaces and capitalization is applied.

The utility is that the resulting rubric can be distributed to students as is.