lint(track_config): check the length of some strings (#335)
lint(track_config): check the length of some strings (#335)

With this commit, `configlet lint` now checks that the value of each of
the below keys in a track's root `config.json` file is at most 255
characters long.

- language
- slug
- exercises.concept[].slug
- exercises.concept[].name
- exercises.practice[].slug
- exercises.practice[].name
- concepts[].slug
- concepts[].name

See also:
- https://github.com/exercism/docs/pull/133, which made these additions
  to the spec

exercism/configlet