Sankey code review

Hello,

I would like to ask for a code review on the Sankey feature.  I've

tried to improve the formatting and documentation. Here is the
comparison:

[https://github.com/kdavies4/matplotlib/compare/master...sankey3#diff-1](https://github.com/kdavies4/matplotlib/compare/master...sankey3#diff-1)

Thanks!

Kevin