Friendly subtree fork of sankey plugin

[soxofaan]

Hi all

I'm using the sankey plugin in a project, and need some improvements (e.g. #73, #1, ...)
Development/maintenance of the d3-plugins repo has stalled (e.g. see https://twitter.com/mbostock/status/600670626785792000), so the pull requests I'd like to depend on probably won't happen soon/ever.

Because I need to move forward, I decided to already start a friendly (subtree) fork of the sankey plugin at https://github.com/soxofaan/d3-plugin-captain-sankey
(I prefixed the name with "Captain" to avoid collision with the future "official" project)

Feel free to submit your pull requests against my repo
or deprecate it in favour of a more "official" one