lint: Add stub implementation

[ErikSchierboom]

This PR is based off #106, so it should only be merged after that PR is merged.

In this PR, a minimal stub implementation of the lint command is added. My goals with this PR are as follows:

• Output a friendly error to tracks when they try to run configlet lint (which will be sooner rather than later)
• Let tracks know that we're busy implementing the linting rules and that they thus need to re-run the check regularly
• Provide a base for others to also start working on the linter. Admittedly, this doesn't help much at the moment, but it is a start.

I've deliberately not added any actual linting rule implementations or tests.

[ee7]

Approved, but we could add something about the WIP-status of lint to the README in the next PR.

The output of ./configlet --help regarding the lint options is a bit weird, but fine for now:

Usage:
  configlet [global-options] <command> [command-options]

Commands:
  lint, sync, uuid

Options for lint:

Options for sync:
  -e, --exercise <slug>        Only sync this exercise
  -c, --check                  Terminates with a non-zero exit code if one or more tests are missing. Doesn't update the tests
  -m, --mode <mode>            What to do with missing test cases. Allowed values: c[hoose], i[nclude], e[xclude]
  -p, --prob-specs-dir <dir>   Use this `problem-specifications` directory, rather than cloning temporarily
  -o, --offline                Do not check that the directory specified by `-p, --prob-specs-dir` is up-to-date

Options for uuid:
  -n, --num <int>              Number of UUIDs to generate

Global options:
  -h, --help                   Show this help message and exit
      --version                Show this tool's version information and exit
  -v, --verbosity <verbosity>  The verbosity of output. Allowed values: q[uiet], n[ormal], d[etailed]

[ErikSchierboom]

Agreed. Will be good to add something to the README.