Create a new command that fetches global track files to the root directory

[kytrinyx]

We have an endpoint in the API, http://x.exercism.io/v3/tracks/:id/global, which delivers a zip file containing a directory tree that should be placed as-is in the root of the track's exercise directory on the user's local machine.

I'd like to have a separate command that people can run, and we will update the documentation for how to get started in a track to tell people to run that command.

For example:

$ exercism setup TRACK_ID

[kytrinyx]

Closing this in favor of #418, which will implement this for nextercism.