added index-support for mk2

For MK2 devices, you can set an index (which side of the blink(1) should be the target).

From the [documentation](https://github.com/sandeepmistry/node-blink1/blob/master/README.md):

![mk2_index](https://cloud.githubusercontent.com/assets/912344/4720889/79bebc88-5934-11e4-89a1-940ae485a34a.png)

I added support for this index. My grunt-config now looks something like this:

```
    blink1: {
      bad: {
        colors: ['red', '#000', 'red'],
        options: {
          ledIndex: 2,
          fadeMillis: 100
        }
      }
    }
```

I can't test whats happening with mk1-devices, because i have none.