Improve migrator workflow

Changes made:
- Added validation of configuration file
     1. User must specify at least one configuration between (all, blacklist, whitelist)
     2. User must not specify both blacklist and whitelist
- Added user confirmation step inside the migrate:up method to confirm addition of the guid column to each model it is being added to
- Updated backfill method to skip rows where the guid is not nil

Todo:
- Prompt user to include autoguid on models that have the guid column and to run the backfill task
- Drop the guid column from models which no longer exist on the whitelist
- What happens if the user includes a model in the config that doesn’t exit in the rails app