Add upgrade command

[oubiwann]

Not sure if this is necessary, since this is provided by rebar3. However, one thing I have noticed with other rebar3 plugins is that the downloaded cache for plugins is preserved and it can be tricky to ensure that the latest version is actually downloaded from a repo. Perhaps the upgrade command only cleans the cache when an actual release is identified? Dunno. This need to be examined in detail, with any missing use cases identified.