Update

Initial parts of determining the current installed version of an app and a hint whether an update is available or not.

Next step would be to implement automatic updating with `brew cask update` or hook into `brew update`. Besides this, the `installed?` command must return `true` even when an old version of an app is installed (might have side effects?).

Posting this as a PR just to discuss it and to determine whether it is going into the right direction.