feature request: list available exercises for download through cli

[scmbradley]

Here's the usecase I'm imagining. I type exercism list (or exercism --track=java list for example) and the cli would output a list of the names of the exercises I have available to download, together with their difficulty level and maybe a short description. This would make things easier.

Perhaps I could then filter this list with e.g. exercism --difficulty=medium list for example...

[arlm]

I would love to have a flag to filter like the status:

• --exercise-status= all, started, iterated, completed, published
• --mentoring-status= any, no-mentoring, mentoring-requested, mentoring-in-progress, mentoring-completed
• --sync-status= all, up-to-date, out-of-date
• --tests-status= all, passed, failed, error, exception
• --latest-tests-status= all, passed, failed, error, exception

Maybe even a --status flag that not only lists but shows the status of the exercises.