1.7.0

• changed step_bestNormalize to step_best_normalize, responding to 8
• Fixed error in documentation regarding LambertW transformation types
  (thank you to Georg M. Goerg, the author of LambertW, for pointing this out).
• Add center_scale transform as default when standardize == TRUE
• Added error when trying to use repeated CV with much too small of folds
• Changed a few T and F to TRUE and FALSE
• Added documentation of how one can use scales and ggplot2 to visualize all transformations.
• Added butcher and axe functionality in order to improve scalability of step_* functions
• Improved tidy functionality with bestNormalize and step_best_normalize