add common angle option to alluvium & flow geoms

[corybrunson]

I'm still vacillating on this but want to articulate it: The common angle plots of @heike and Vendettuoli, implemented in {ggparallel}, individualize the corner position in order for linear ribbons to share the same slope, preventing the line width illusion (due to fixed-height ribbons having different widths depending on slope).

For alluvial plots, this would generalize to any style of curve by individualizing the width position aesthetic for the lodes (and for the gaps between the flows) while leaving it fixed for the strata. Rather than yet another Geom* parameter, might this be an appropriate job for a new Position* ggproto?