Convenience function for generating crosstabs using `set_table()`

We want to be able to generate a crosstab easily in the following way (taken from Danny's comment on #2):

```r

set_table(
  mtcars,
  .rows = cyl,
  .cols = crosstab_freq(vs)
) %>%
project_table()


set_table(
  mtcars,
  .rows = cyl,
  .cols = list(
    crosstab_freq(vs),
    col_median(blah)
  )
)
```