gg_miss_case add a `pct` option as in gg_miss_var

[njtierney]

So that there is more consistency between the two functions

[njtierney]

Here is the current output (V 2.2.9100)

library(naniar)
gg_miss_case(airquality)

gg_miss_case(airquality, show_pct = TRUE)

# extend the ylimits
library(ggplot2)
gg_miss_case(airquality, show_pct = TRUE) + 
  ylim(0, 100)

Created on 2018-06-05 by the reprex package (v0.2.0).