File structure for challenges in user environment

[joeljuca]

Dsafio will download challenges from GitHub and place them somewhere in the user file system. We should describe a file structure to store challenges in users' disks.

[joeljuca]

The Exercism approach (<prefix>/<challenge-name>) was suggested in #18. E.g.:

- <dsafio-prefix>/ (e.g.: ~/dsafio)
  - <challenge-name>/ (e.g.: hello-world)
    - index.js
    - readme.md
    - test.js

[joeljuca]

The Exercism approach will be used as starting point.