Implemented option allowing to specify Donut segment color in data parameter

Hi,
I'm displaying categories with specific color, so I needed way to tell Morris color of each segment. To set segment color just add field called `color` to data item.

I believe this might be useful to someone else as well.

JSbin: http://jsbin.com/UcUBApA/3/

Example:
```JavaScript
Morris.Donut({
    element: 'donut-example',
    data: [
      {
          label: "Download Sales",
          value: 12,
          color: "#5a3163"
      },
      {
          label: "In-Store Sales",
          value: 30,
          color: "#894a96"
      },
      {
          label: "Mail-Order Sales",
          value: 20,
          color: "#ae73ba"
      }
    ]
});
```