Illustrate combinations with aesthetic-focused ggplot2 extensions

[corybrunson]

Two recent issues, #120 and #134, involve problems making {ggalluvial} work with other {ggplot2} extensions. A new vignette should provide illustrations for these two cases (once the former is resolved) and general guidance on the kinds of problems one might face in other settings (due, e.g., to the non-standard use of the group aesthetic).