Add argument for one hot encoding to workflows

[juliasilge]

This PR integrates the one_hot argument from tidymodels/parsnip#332 and tidymodels/hardhat#140 into workflows. After all this work, finally, FINALLY, the weirdness demonstrated in tidymodels/parsnip#326 is resolved:

library(parsnip)
library(workflows)

lm_spec <- linear_reg() %>%
  set_engine(engine = "lm") 

lm_spec %>%
  fit(Sepal.Length ~ ., data = iris)
#> parsnip model object
#> 
#> Fit time:  5ms 
#> 
#> Call:
#> stats::lm(formula = Sepal.Length ~ ., data = data)
#> 
#> Coefficients:
#>       (Intercept)        Sepal.Width       Petal.Length        Petal.Width  
#>            2.1713             0.4959             0.8292            -0.3152  
#> Speciesversicolor   Speciesvirginica  
#>           -0.7236            -1.0235

workflow() %>%
  add_model(lm_spec) %>%
  add_formula(Sepal.Length ~ .) %>%
  fit(data = iris)
#> ══ Workflow [trained] ══════════════════════════════════════════════════
#> Preprocessor: Formula
#> Model: linear_reg()
#> 
#> ── Preprocessor ────────────────────────────────────────────────────────
#> Sepal.Length ~ .
#> 
#> ── Model ───────────────────────────────────────────────────────────────
#> 
#> Call:
#> stats::lm(formula = ..y ~ ., data = data)
#> 
#> Coefficients:
#>       (Intercept)        Sepal.Width       Petal.Length        Petal.Width  
#>            2.1713             0.4959             0.8292            -0.3152  
#> Speciesversicolor   Speciesvirginica  
#>           -0.7236            -1.0235

^{Created on 2020-06-23 by the reprex package (v0.3.0.9001)}

🙌

[juliasilge]

I just set DESCRIPTION to use development versions of parsnip + hardhat, but neither of those PRs are merged as of right now.

[juliasilge]

It will be super important to redocument after merging this because of the material in man/rmd/indicators.Rmd, which currently has conflicting info (5 vs. 6 predictors) due to the one hot issue.

[github-actions]

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