Add .keep to mutate()

[nathaneastwood]

This argument allows you to control which columns from .data are retained in the output:

• "all", the default, retains all variables.
• "used" keeps any variables used to make new variables; it's useful for checking your work as it displays inputs and outputs side-by-side.
• "unused" keeps only existing variables not used to make new variables.
• "none", only keeps grouping keys (like transmute()).