Fix drawPolygon's fill and stroke colors not working

Summary: Looks like commit 301a1cf6 accidentally `_.extend`ed
incorrectly, and so stroke color and fill color were always set to the
defaults. Also, we didn't substate khan-exercises in graphie-to-png for
a long time, so nobody noticed it was broken for a while.

Test plan:
 - Call within a khan-exercise and make sure that it works:
```
addMovablePolygon({
    fixed: true,
    points: [[1, 1], [4, 1], [4, 4], [1, 4]],
    normalStyle: {
        stroke: PINK,
        fill: PINK,
        "fill-opacity": 1
    }
});
```
 - Make the same change within the graphie-to-png khan-exercises, see
 that this works:
```
drawPolygon({
    points: points,
    arcs: 1,
    ticks: 1,
    stroke: PINK,
    fill: PINK,
    "fill-opacity": 1
});
```

Auditors: aria