[FEATURE] Support vagrant global-status

Vagrant 1.6 introduced the `global-status` command which allows
to get a quick overview of all active Vagrant environments for the
currently logged in user.

plugins/vagrant/_vagrant

@@ -8,6 +8,7 @@ _1st_arguments=(
     'box:Box commands'
     'connect:Connects to a shared, remote Vagrant environment'
     'destroy:Destroys the vagrant environment'
+    'global-status:Reports the status of all active Vagrant environments on the system.'
     'halt:Halts the currently running vagrant environment'
     'init:[box_name] [box_url] Initializes current folder for Vagrant usage'
     'login:Authenticates against a Vagrant Cloud server to access protected boxes'