Added series index params in pie labelFormatter

Hi, 

First of all I'd like to thank you for creating such a wonderful lib. I am using flotr2 to develop a website that contains many kinds of chart.

However, I found that there's one thing that can't satisfy my requirement - that is, I want to show some extra info in pie chart's label. Currently `labelFormatter` can only show the total value and current value. I added some data in the series I passed to flotr, but the pie chart just cannot access them. 

I think I can show more info if I know the index of the pie slice. So I just add a `count` attribute, so that it can tell me what the index of the current part.