Implement default IP resolving as a `ip_resolver` Proc

[pbitty]

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I'd like to have the IP resolving logic always use the ip_resolver Proc, and implement the default behavior (of using private_network or ssh_info) as a Proc itself. This way we can eventually have built-in resolvers for different use-cases.

For example, as in #86, reading an interface's ip address in a DHCP scenario could look something like this:

config.hostmanager.ip_resolver = :dhcp, interface: "eth1"

This would find the dhcp resolver and instantiate it with the option interface: "eth1".

Before implementing this, I'd like do some refactoring as per #99. I'm just putting this here so it's not forgotten.