Allow datum index to be passed as a 3rd parameter to the data labels function

Hi @masayuki0812, thanks for maintaining this great project.

I made this change because there may be times when we need to change the data label text according to the index. I hope it makes sense. [Here](http://jsfiddle.net/edcartre/wxhfwvfg/) you can find a jsfiddle with the results.