Future scope: identify which extensions can be updated (e.g. pie update)

[asgrim]

From discussion php/pie#31 (comment) with @kynx :

OT, but the big thing for me is to have tooling like renovate recognise when extensions need updating.

There will be a pie show command according to the design initially, but a nice improvement to this would be a way of indicating if a newer version is available (and perhaps show which version, compatible semver, and also "latest").