Add subnet verbose mode

[noqqe]

I added a verbose mode for subnet command.

> go run main.go -v sub 192.168.178.0/24 4
Prefixes: 192.168.178.0/26
  Addresses:  64
  Netmask:    255.255.255.192
  First:      192.168.178.0
  Last:       192.168.178.63

Prefixes: 192.168.178.64/26
  Addresses:  64
  Netmask:    255.255.255.192
  First:      192.168.178.64
  Last:       192.168.178.127

Prefixes: 192.168.178.128/26
  Addresses:  64
  Netmask:    255.255.255.192
  First:      192.168.178.128
  Last:       192.168.178.191

Prefixes: 192.168.178.192/26
  Addresses:  64
  Netmask:    255.255.255.192
  First:      192.168.178.192
  Last:       192.168.178.255