Add IPAddr#+/-

[taketo1113]

Add +/- methods to get an ipaddr instance greater/less than the original address by offset. the current IPAddr#succ only return next address, this IPAddr#+ return any address with offset.

addr = IPAddr.new("192.168.1.10")

addr + 10
=> #<IPAddr: IPv4:192.168.1.20/255.255.255.255>


(addr + 10).to_s
=> "192.168.1.20"