Add colors to pretty formatter

Add colors to pretty formatter and summarizer (optional), using the commonjs-ansi-color dependency.

@jbpros This still needs tests, but sending your way to see if you have comments on the approach first. Current tests pass.

- Added color constants to Formatter. Not sure if they belong there, but using them from Summarizer, so needed a common place between that and PrettyFormatter.
- Added colorizer function to Formatter, too.
- See Summarizer for a private adorn() function. Could also be put in a common place.