added simple path to external labels functionality on pie charts

[alan-unravel]

Requires externalLabels to have a value. Should probably also require radius otherwise the labels will get cut off very frequently. Not as fancy as the initial example, will have to work on that

[gordonwoodhull]

centroid is never used.

[gordonwoodhull]

Thanks @alan-unravel, I am merging this and making a few improvements:

• implementing the transition cases
• removing the unused centroid noted above
• applying the arc to the interpolated angle rather than the final angle
• making lines to zero-sized wedges invisible

As you can see on this test page, there are many problems with pie transitions:
http://dc-js.github.io/dc.js/transitions/pie-external-label-transitions.html
.. but these path transitions are mostly better behaved at this point.

I think the position of the line could still be improved. Right now it's drawing from the center of the wedge to the center of a ring segment with the label on the outer radius. IMO it would be nicer to draw it directly to the label, but clip it at the label or a slight margin away from the label. I have code for that elsewhere but this is good enough for now.

[gordonwoodhull]

Merged in 2.0.0 beta 20

[alan-unravel]

Thanks @gordonwoodhull let me know if I can be of any help, the original lines from the example I put in gitter were nice but I didn't have the time to generalize the concept.