Add onAnimationProgress callback function

In my case, I am doing a simple doughnut pie chart with only 2 data values. I wanted to display the percent complete of one of them as the animation was in progress.

I realize the library isn't made for interactiveness, per se -- but in addition to the already existing onAnimationComplete callback, this simple addition just provides insight to the percent of the animation completed. I think it serves as a good fit with the current config options.

it would just be another option used like:
```
onAnimationProgress: function(progress) {
    console.log(progress); // 0 to 1
}
```