Add command to update all outdated plugins

Command `:plugin-manager.update-outdated`.
Behaviors are reloaded only after the install process for the last
plugin is completed; the user is also notified of what has been updated.

Caveat: notifos events from `:behaviors.reload` may override the update
list notification.
Merge pull request #1331 from ndr-qef/update-outdated

Add command to update all outdated plugins