Directed arrows for Sankey Network

[matt-sd-watson]

Aims to resolve #277. The Sankey network now includes the arrows = T option (F by default) to add directed arrows within the link bar between nodes with opacity darker than the link bar to indicate the connections. As of now the arrowhead is not placed at the end of the link bar, and some angles do not work perfectly. I may need to work on this further but it is a step towards a full solution.