argument for retaining original in `step_dummy`

[EdwinTh]

As also noted in the Description, step_dummy removes the original variable. I think it would be valuable to have the option to retain it. Personally, I always keep keep my characters/factors in the modelling set to do post-model analyses.

Happy to do a PR, please let me know how you feel about this.

[topepo]

Changes have been checked in if you want to test

[EdwinTh]

Great, does look good to me!

[github-actions]

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