Improve Corner Colors - BLANK enum and restore on reset

[sjtrny]

Change Summary:

1. Color definitions have been centralised
2. Modified the BLANK constant to have a non None value to resolve equality checks
3. The world corner_colors dictionary is now auto-filled with BLANK when corner color not provided
4. Painting BLANK corner color is now supported
5. Initial corner color state is restored when calling reset_world

Added associated tests for new functionality.

This PR resolves the following issues:

1. https://github.com/TylerYep/stanfordkarel/issues/74
2. https://github.com/TylerYep/stanfordkarel/issues/75