Ability to Define DataView Columns for Labels

In order to define labels, you have to create a dataView: https://developers.google.com/chart/interactive/docs/gallery/barchart#Labels

The change would allow you to set dataViewColumns on the directive's chart object in the same way: 
```javascript
 chart.dataViewColumns = [0, 1,
                       {
                           calc: "stringify",
                           sourceColumn: 1,
                           type: "string",
                           role: "annotation"
                       }];
```

I can work on providing the funcitonality in a different way (perhaps another scope variable in the directive) if you think there's a better solution.