Feature `one_hot` argument for `step_dummy()`

[DavisVaughan]

Since I sadly deleted the PR #136 branch with one_hot, I could not reopen it and have reimplemented it here, incorporating your comments from there.

I have not removed the contrasts argument, as I feel that is a separate feature.

[DavisVaughan]

Regarding the contrasts argument, trying the code below with all 5 of the different unordered contrast types set as global options results in behavior mimicking a fallback to using the default, contr.treatment, with one_hot = TRUE. This probably makes sense right?

data(okc)
okc <- okc[complete.cases(okc),]

rec <- recipe(~ diet + age + height, data = okc)

rec %>% 
  step_dummy(diet, one_hot = TRUE) %>%
  prep(training = okc) %>%
  bake(newdata = okc)

I think the bit of model.matrix() that determines what to do in these cases happens at the C level, I'm not exactly sure where, but maybe this bit has something to do with it.

[topepo]

Regarding the contrasts argument, trying the code below with all 5 of the different unordered contrast types set as global options results in behavior mimicking a fallback to using the default, contr.treatment, with one_hot = TRUE. This probably makes sense right?

It does. I added to the dummy variable vignette to mention this.