There should be an option to turn off `skip()` in a lesson

[cansavvy]

For when a teacher is using swirl for a custom course, sometimes they are using it to give credit for a student having done the work. Apparently skip() can allow students to skip questions in any lesson which means a student could skip through the whole lesson and still get credit.

It would be great if in swirlify there was a way to turn off "skip()" in a customized lesson.

This might look like providing an option in the lesson.yaml or customTests.R. Not sure at this time what the solution would look like.