Added support for shading between curves

Summary:
Previously, plotParametric supported shading the area under a given function. This commit
generalizes that behavior to allow for shading between two user-provided functions. There might be
a simpler way to do this using Raphael, but currently, plotParametric looks for points at which the
two functions intersect and then shades the area between them.

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

Differential Revision: http://phabricator.khanacademy.org/D10098