get back on track with official d3-sankey

The official [d3-sankey](https://github.com/d3/d3-sankey) library has finally gotten some love recently, it even gained an official [horizontal alignment](https://github.com/d3/d3-sankey/pull/4) feature. Should we consider getting back to tracking the official version? Frankly, I have avoided messing with any of the sankey code here because so many minor changes/additions have been made I'm afraid of messing something up, but we would likely lose some current features.

Maybe we/someone can audit what we have and compare it to the official library and make a list of additional features we have here, so we can assess the cost of losing them or reimplementing them?