Feature Request: Show hints as requested

It would be really great if exercises show incremental hints. So for example, the darts exercise is a bit tricky cause it asks if the point where the dart hit is within the various point circles. One may not realize that it's not easy as looking at the coordinates of the point to figure out that the point is in a particular circle. I myself fell for this and thought I could just follow the largest point to see if it is beyond the limits of the circle, but that assumes it is a square and not actually a circle. In this case, I think it would be useful to let a student struggle to figure out this intricacy, but in order to not lose students due to frustration in understanding the problem, in this case it would be helpful if they click a `Get Hint` button and it could show a diagram of how the circles would look to help them understand their approach better.


Each subsequent `Get Hint` click would show additional pre-determined hints until those are all exhausted. This wouldn't be necessary for all exercises, but rather on especially tricky problems.

Thoughts?