Add UUID verification to the lint command

This release adds three pieces of behavior to the lint command:

• verify that each exercise, including deprecated, has a defined UUID within its track's config.json.
• verify that each exercise has a unique UUID within its track's config.json; outputting one lint error for each set of duplicated UUIDs.
• verify that each exercise has a unique UUID across all of Exercism's tracks, by remotely verifying all exercise UUIDs against the Exercism UUID verification service.

The remote, HTTP-based, UUID verification process can be disabled at runtime with the --no-http flag.

$ configlet lint path/to/track --no-http