Toggle between percentage and absolute values

Hi, I have made a new property which makes it possible to toggle between "Absolute" and "Percentage". As I am working with your cohort library I needed this function, but it wasn't been implemented. 

I created a dirty hack by looping at at the table after it was rendered. That's not really the best option, so I decided to create a better "fix". Could you please merge this with your master. 

Please don't blame if the test case isn't working. I have never worked with PhantomJS/Mocha before and I don't have that installed right now. 
At least I tried to :smile: 

Thanks!
since `formatPercentage(value, baseValue)` is being used twice, I'd prefer to have this in a variable and reuse it.
Something like:

```js
var value = row[j],
  formattedPercentage = formatPercentage(value, baseValue),
  cellValue = j === 0 || config.displayAbsoluteValue ? value : formattedPercentage,
  opts = {};
```