Added support for providing 1 or 2 extra colours to the contour routines to easily specify the under and over colors.
@efiring's suggestion.
Added an api_changes entry for contour levels & colours with the extend keyword set.
Merge pull request #2002 from pelson/contour_levels_with_colors

Added support for providing 1 or 2 extra colours to the contour routines to easily specify the under and over colors.