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

## 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.