Add a new 'overview' command line option to make the output more
compact and display only properties that have non-default
values. Currently the option has only effect for the "connection show
$CON" sub-command.

 $ nmcli -o connection show wifi-home
 connection.id:                          wifi-home
 connection.uuid:                        8308c425-f2a7-4021-9afc-37bde7253c6d
 connection.type:                        802-11-wireless
 connection.timestamp:                   1519264421
 connection.permissions:                 user:me
 802-11-wireless.ssid:                   home
 802-11-wireless.mode:                   infrastructure
 802-11-wireless-security.key-mgmt:      wpa-psk
 802-11-wireless-security.auth-alg:      open
 ipv4.method:                            auto
 ipv6.method:                            auto

https://bugzilla.redhat.com/show_bug.cgi?id=1434527