Add command to update all outdated plugins

A simple command that checks for outdated plugins and updates them sequentially. I am under the impression that a markup refresh for the Plugin Manager is due relatively soon, so I have not added any GUI as requested in #1275.

Before merging, I would like to add some sort of user notification and compound the callbacks. Is there an elegant way to only act upon the last callback from `discover-deps`, or would storing the count in an atom be an acceptable solution?