Added a 4th param to data label format callback

Thanks for merging my last PR! (#675)

Recently I've discovered that there's a third argument in d3's callbacks in case of nested selections as shown in [this Mike Bostock's article](http://bost.ocks.org/mike/nest/). So this is a new PR to implement this tiny but useful option (at least for the kind of applications I need).

You can view the results in my [updated fiddle](http://jsfiddle.net/wxhfwvfg/5/)

Keep up the good work. C3 is getting better all the time!