Added a template for shading area between two curves

Summary:
Now that graphie.js has the ability to shade the area between curves, this commit both
updates khan-exercises and adds a template that demonstrates how to shade the area between two
curves using graphie. Fixes T2686.

Test Plan:
- Run graphie-to-png.
- Try out "40: Shade area between curves" to see the new behavior. Play around with the fn1 and fn2
variables.
- Also try out: "37: Shade area under curve" to check that previous behavior remains the same.

Reviewers: jack, eater

Reviewed By: eater

Subscribers: eater

Maniphest Tasks: T2686

Differential Revision: http://phabricator.khanacademy.org/D10099