minor improvements to `gg_miss_fct`

[njtierney]

Some minor improvement to gg_miss_fct- would be adding a minimal theme (instead of gray) and angled text on the x axis.

There needs to be some way to deal with suuuuper long x labels, but I'm not sure what that is.

library(naniar)
library(ggplot2)

gg_miss_fct(x = riskfactors, fct = marital)

gg_miss_fct(x = riskfactors, fct = marital) + 
  theme_minimal() + 
  theme(axis.text.x=element_text(angle=45,hjust=1))