enable transpose when with multiple `by` strata.

[DanChaltiel]

mtcars2 %>% 
  crosstable(mpg, by=c(am, vs)) %>% 
  t()

mtcars2 %>% 
  mutate(am_vs = paste(am, vs, sep="__")) %>% 
  crosstable(mpg, by=c(am_vs)) %>% 
  t() %>% 
  separate(am_vs, into=c("am", "vs"), sep="__")