update

• dplyr 0.8.0. compatibility
• vdiffr is now used to test plots and added as a suggested dependency
• manip_bin_numerics() accepts c('median', 'mean', 'cuts', 'min_max') as bin_labels
  argument which will be converted to bin label.
• alluvial_wide() and alluvial_long() do not crash anymore when dataframes are grouped