Add function/vignette for conditional plotting based on missingness indicator

[Hanne Oberman]

E.g.

ggmice(boys, aes(age)) + 
  geom_histogram() + 
  facet_wrap(vars(is.na(gen)))

[Hanne Oberman]

Probably nice to add

ggplot2::update_geom_defaults("bar", list(fill = "white"))

to theme_mice() then

[Hanne Oberman]

Fixed in 556ede9