Allow fixing start and finish colors in Ordering.
Extract color double setup in to shared context.
Turn on treat_symbols_as_metadata_keys_with_true_values
Add ClosestPairFinder.

To find the closest pair of colors in a pair of groups.
Add ClosestPairFinder.pair method.
Add GroupedSort.

This is a very rough version.
Add GroupSorter and ColorSort.sort_groups.
Refactor the grouped sort code.
Update README with instructions on sorting groups.