3 new methods in Cars.rb

1. color= setter
2. class method most_popular_color
3. color_into_hash — originally part of initialize, but now a separate
method that is called by initialize and color=
Update initialize and new Convertible subclass

1. updated initialize on Car with a default value of :unknown for color
2. made a new subclass Convertible with the following methods:
initialize, top_down, close_top, to_s
3.  The Convertible subclass has an instance variable @roof_status
which is either :open or :closed.
4.  Used super for part of initialize and to_s methods