Sankey improvements

[mbostock]

The Sankey plugin could use some improvements, such as options for disabling the move-sinks-right and enabling move-sources-right. Also, being able to set the x-position of the nodes in a Sankey diagram would be useful for alluvial diagrams; it might also be useful to control the y-order of nodes for a given x-value.

[thiloplanz]

For manually setting the x-position, see #88

[soxofaan]

FYI: I merged #91 in the master branch of my subtree fork of the sankey plugin at https://github.com/soxofaan/d3-plugin-captain-sankey (also see #133 )