Handle argument as IPAddr if possible

Some providers have ranged IP addresses for their services.
For example, github.com has [192.30.252.0/22](https://help.github.com/articles/what-ip-addresses-does-github-use-that-i-should-whitelist/).

I want to write spec like that:
```ruby
        expect('github.com').to have_dns.with_type('A')
          .and_address('192.30.252.0/22')
```