add facts showing available updates

Making use of the apt-check command from the 'update-notifier-common'
package (if available) display the number of available updates, number of
security updates as well as the update package names.
Because `.split()` returns an array this will end up being a structured fact for people that run with Facter 2 and `stringily_facts` disabled but what's the behaviour on Facter 1.7?