text: Title: modify _find_or_check_categorical_variables to support dtype='category'
  Body: **Is your feature request related to a problem? Please describe.**
  Currently, _find_or_check_categorical_variables in variable_manipulation.py only checks for type object to determine categorical variables.
  
  **Describe the solution you'd like**
  The method should be enhanced to support dtype="category" to support cases where user has used category datatype to optimise memory footprint of Pandas dataframe. 
  (https://pandas.pydata.org/pandas-docs/stable/user_guide/categorical.html)
  