Support simple arithmetic expression with df.variable or aesthetic

[alshan]

For example :

• fill = change < 0 https://stackoverflow.com/questions/59989254/different-bar-color-for-positive-and-negative-bars-in-bar-plot
• fill=factor(ifelse(area=="Południe","Highlighted","Normal")) https://stackoverflow.com/questions/22894102/change-color-of-only-one-bar-in-ggplot
• color=ifelse(df$y<0, 'red', 'black') https://stackoverflow.com/questions/15804504/how-to-conditionally-highlight-points-in-ggplot2-facet-plots-mapping-color-to
• https://community.rstudio.com/t/ggplot-with-conditions/17352

[alshan]

Related: #1008