Sankey with end links

Updated version of d3-sankey-path library, with standalone library to append arrows.

This also includes an experiment with allowing circular links from end nodes to go either top or bottom, and always follow the same direction as the other links connected to those nodes.

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forked from tomshanley's block: Sankey with circular links

forked from tomshanley's block: Sankey, with different end note link treatment

forked from noblemillie's block: Sankey, with different end note link treatment