improve compare_diffs()

[oharac]

Improve fucntionality of compare_diffs():

• include a check to make sure comparator (lhLessons package) is up to date with the most recent
• exclude diffs for s00_ files - use this prefix for starting matter for a course
• eliminate dependency on metadata_course.csv and metadata_lessons.csv - these are generated at the initial setup, but don't get auto updated - kind of a pain to update!