Add support for specifying range of IPs

[atc0005]

This is desirable for the "allow" list of IP Addresses which should not be subject to user account disabling/blocking behavior. The current design supports only a single IP Address, whereas for real production use specifying an optional range may be needed.

refs:

• https://stackoverflow.com/a/40072896/903870
  • https://play.golang.org/p/u6JBAlIBLa
• https://golang.org/pkg/net/#IPNet.Contains