Add "outdated" and "update" commands

[sdegutis]

• "outdated" would show packages that have available updates.
• "update" would update the package to its newest version.

[hishamhm]

Hi, I just added luarocks list --outdated.

Please check it out!

The luarocks.list module now has a check_outdated function. It is local for now (and its arguments are not designed to be nice for use from the outside world, but to integrate well with the various query styles of the luarocks list function), but a new public function could be made that returns a table with all outdated rocks, if you'd like to integrate that with your UI. Let me know how that sounds!

[sdegutis]

Thanks!