Description:
    Generates a migration of database content. 
    
    To import database content from a json file move your json file to db/migrate_cards/data
    and choose as migration name import_<filename>.
    
Example:
    wagn generate card_migration better_commented_search_cards

    This will create:
    
        db/migrate_cards/(migration_number)_(name).rb

    ...which is set up to use methods in lib/wagn/migration.rb
    
    
    wagn generate card_migration import_better_search_cards

    This will create:
    
        db/migrate_cards/(migration_number)_(name).rb
        
    ...which is set up to import data from db/migrate_cards/data/better_search_cards.json