Add coverage for Quiz Extensions

[Thetwam]

We recently released the Quiz Extensions tool. The tool is a bit older and predates the CanvasAPI library. To clean up the code for that project, we have an issue open to start using CanvasAPI (ucfopen/quiz-extensions#2).

However, we do not yet have coverage for the Quiz Extensions endpoint in CanvasAPI.

https://canvas.instructure.com/doc/api/quiz_extensions.html

There is also a Course Quiz Extensions endpoint. Although the Quiz Extensions tool doesn't use this endpoint, it's similar enough to warrant adding it as well.

https://canvas.instructure.com/doc/api/course_quiz_extensions.html

Add both of these endpoints to CanvasAPI.