feature request: list available exercises for download through cli

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...