OHE: User can select which categories to encoded for selected variables

[Morgan-Sell]

Closes #303.

Objective: Create functionality so a user can encode certain categories that may not be the most frequent. The functionality is explained in this thread.

Will create a new init param called custom_categories that accepts a dictionary with selected features as keys and lists of the desired categories as values.

Both top_categories and user_categories cannot be used at the same time.