Add dual-stack assignment to CLI

This adds dual-stack assignment to the CLI so that both an IPv4 and
IPv6 prefix can be assigned when assigning from a pool. It is not
possible to specify the prefix-length for the new prefix per family,
thus it doesn't make much sense to allow it at all.

Fixes #526.
Merge pull request #542 from plajjan/526-cli-dual-stack

Add dual-stack assignment to CLI