* core: allow all CIDR ranges in zone specifications

Allow (e.g.) a v4 reverse on a /17. If a zone is specified in such a
way a FilterFunc is set in the config. This filter is checked against
incoming queries.

For all other queries this adds a 'x != nil' check which will not impact
performace too much. Benchmark function is added as well to check for
this as wel.

Add multiple tests in tests/server_reverse_test.go.

Benchmark shows in the non-reverse case this hardly impact the speed:

~~~
classless:
pkg: github.com/coredns/coredns/core/dnsserver
BenchmarkCoreServeDNS-4   	 1000000	      1431 ns/op	      16 B/op	       1 allocs/op

pkg: github.com/coredns/coredns/core/dnsserver
BenchmarkCoreServeDNS-4   	 1000000	      1429 ns/op	      16 B/op	       1 allocs/op

master:
pkg: github.com/coredns/coredns/core/dnsserver
BenchmarkCoreServeDNS-4   	 1000000	      1412 ns/op	      16 B/op	       1 allocs/op

pkg: github.com/coredns/coredns/core/dnsserver
BenchmarkCoreServeDNS-4   	 1000000	      1429 ns/op	      16 B/op	       1 allocs/op
~~~

* README.md updates