Make it easy to change the default size of a string/varchar field

This make sit easy to overwrite the default string column length for ActiveRecord migration. By running `require 'active_record/database_validations/varchar_191'`, it will overwrite the default in the inner workings of ActiveRecord to use `VARCHAR(191)`. `require 'active_record/database_validations/varchar_255'` will restore the default to 255.

@byroot @singerwang for review