Allow shading the area under a graphie-plotted function

Summary: Shading a parametric equation perhaps doesn't make a ton of sense by itself, but maybe it'll be useful.

Test Plan:
In graphie-to-png:

```
plot(function(x) {
    // Function to shade under
    return 0.03 * (x+5)*(x+2)*(x-3)*(x-6);
}, [-5, 3], // <-- x-range to shade
    false, true, { stroke: BLUE, fill: BLUE, opacity: 0.3 }
);

plot(function(x) {
    // Plot the entire function
    return 0.03 * (x+5)*(x+2)*(x-3)*(x-6);
}, X_RANGE);
```

Reviewers: jack

Reviewed By: jack

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D8116