feature request: `exercism download` should warn the user if a newer version is available

[andreagrandi]

Scenario

If I run a command like this exercism download --exercise=scale-generator --track=go and I already have a copy of the code downloaded, and in the mean time the upstream exercise has been updated, I won't know if I have the latest version or not.

Proposed solution

Add a version field to the metadata.json. This file should be downloaded first and its value should be compared with the one in the local metadata.json. If the values are different, the user should be warned that a new version is available.