Implement subcommands: lint, generate

As of this release, there is no longer support for the command configlet . or configlet path/to/track. If you want the behavior that was previously accessible via the top-level command, you must now use the lint subcommand explicitly:

configlet lint path/to/track

In addition to the lint command which verifies the track configuration, there is now a second subcommand, generate, which generates READMEs for the exercises in the track.

configlet generate path/to/track [--only $SLUG]

See the documentation for details.