Added --unique, -u for randip

[rm-Umar]

Added a new flag in randip for generating only unique IPs.

./randip -s 127.0.0.0 -e 127.0.0.5 -n 5 -u 
127.0.0.3
127.0.0.4
127.0.0.0
127.0.0.1
127.0.0.2

[UmanShahzad]

Getting an error, seems due to overflow when you do the +1:

$ ./build/randip 
panic: runtime error: integer divide by zero

goroutine 1 [running]:
github.com/ipinfo/cli/lib.RandIP4Range(0xffffffff00000000, 0x0, 0x5cee84, 0xf, 0xffffffff00000000, 0x0, 0x0)
	/home/mslm/dev/ipinfo/cli/lib/ip.go:142 +0x178
github.com/ipinfo/cli/lib.RandIP4RangeListWrite(0x5cd057, 0x7, 0x5cee84, 0xf, 0x1, 0x5d0000, 0x5715a0, 0xc0000ca000)
	/home/mslm/dev/ipinfo/cli/lib/ip.go:195 +0x3e6
github.com/ipinfo/cli/lib.CmdRandIP(0x0, 0x1, 0x1, 0x5cd057, 0x7, 0x5cee84, 0xf, 0x726690, 0x0, 0x0, ...)
	/home/mslm/dev/ipinfo/cli/lib/cmd_randip.go:90 +0x134
main.cmd(0x7ffc79b7cc8d, 0x6)
	/home/mslm/dev/ipinfo/cli/randip/main.go:146 +0x485
main.main()
	/home/mslm/dev/ipinfo/cli/randip/main.go:157 +0x85