and use it for 'nmcli device show'.

This allows filtering output not only for whole groups, but also for individual
fields in commands that print data in groups (sections).

Example:
$ nmcli -f general.device,general.driver,ipv4,ipv6.address device show eth0
GENERAL.DEVICE:                         eth0
GENERAL.DRIVER:                         e1000e
IP4.ADDRESS[1]:                         ip = 10.0.5.228/23, gw = 10.0.5.254
IP4.ADDRESS[2]:                         ip = 5.5.5.5/32, gw = 5.5.5.1
IP4.DNS[1]:                             192.168.122.1
IP4.DNS[2]:                             8.8.8.8
IP4.DOMAIN[1]:                          mycompany.com