nuggets 2.2.0

• fixed bugs in explore()
• added special value highlighting in explore()'s table of rules
• added contingency table view to explore.associations()
• added explore() methods for baseline contrasts, complement contrasts, paired baseline contrasts, and correlations
• added dig_ancestors()
• added plot_contingency()
• enhanced info and error messages
• added arguments for line type and colour to geom_diamond()
• enhanced dig_*_contrasts() - statistical test errors are now stored in the "comment" column in the returned data frame