Add background colors and a script to show color combinations (colors.py)

Modify background colors support introduced in:
    59e9544e68bc77ee9545d317bac4bc94f813259b
No backward compatibility considered, as unlikely someone as used it yet.

Modify tool._ColorFormatter to support background colors.

Add utility to show possible background/foreground combinations, in
preparation for possibility for user to select default use.

Bug: T135984
Change-Id: Iecb648176b41c9ea3a2cc4ce879526f20d75bfde