Support the case of default gateway outside any subnet

[silverwind]

The module currently assumes that the default gateway lies within one of the configured interface subnets but there are cases where this assumption does not hold true.

One example is a Travis VM having 10.20.0.153/32 with gateway 10.20.0.1. Strictly speaking, this is a invalid network configuration but it somehow still works so we need to account for it.

One approach could be to find the address nearest to the gateway. It's a guess but chances are it might be correct. To get the nearest IPv6 adress conveniently, we need to convert both the address CIDR and gateway address to a BigInt and then compare them, so I would like to require native BigInt (node 10.4.0+).

[silverwind]

Published https://github.com/silverwind/ip-bigint to convert IP <-> BigInt.