Fetch exercise

[holandes22]

The relevant endpoints are:

http://exercism.io/api/v1/exercises?key=<api_key>
http://x.exercism.io/v2/exercises/elixir?key=<api_key>
http://x.exercism.io/v2/exercises?key=<api_key> (all)

Not sure how the cli prints the status, but we should show something similar.

Not Submitted:    1 problem
elixir (List Ops) /home/pablo/exercism/elixir/list-ops

unchanged: 1, updated: 0, new: 0

One question here is: fetch the next problem in the selected track or have a global action that fetches for all the tracks, regardless of the selection.