CIDR query routing (#1159)

* 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

core/dnsserver/directives.go

@@ -1,64 +0,0 @@
-package dnsserver
-
-import (
-	"fmt"
-	"os"
-	"strings"
-)
-
-// RegisterDevDirective splices name into the list of directives
-// immediately before another directive. This function is ONLY
-// for plugin development purposes! NEVER use it for a plugin
-// that you are not currently building. If before is empty,
-// the directive will be appended to the end of the list.
-//
-// It is imperative that directives execute in the proper
-// order, and hard-coding the list of directives guarantees
-// a correct, absolute order every time. This function is
-// convenient when developing a plugin, but it does not
-// guarantee absolute ordering. Multiple plugins registering
-// directives with this function will lead to non-
-// deterministic builds and buggy software.
-//
-// Directive names must be lower-cased and unique. Any errors
-// here are fatal, and even successful calls print a message
-// to stdout as a reminder to use it only in development.
-func RegisterDevDirective(name, before string) {
-	if name == "" {
-		fmt.Println("[FATAL] Cannot register empty directive name")
-		os.Exit(1)
-	}
-	if strings.ToLower(name) != name {
-		fmt.Printf("[FATAL] %s: directive name must be lowercase\n", name)
-		os.Exit(1)
-	}
-	for _, dir := range directives {
-		if dir == name {
-			fmt.Printf("[FATAL] %s: directive name already exists\n", name)
-			os.Exit(1)
-		}
-	}
-	if before == "" {
-		directives = append(directives, name)
-	} else {
-		var found bool
-		for i, dir := range directives {
-			if dir == before {
-				directives = append(directives[:i], append([]string{name}, directives[i:]...)...)
-				found = true
-				break
-			}
-		}
-		if !found {
-			fmt.Printf("[FATAL] %s: directive not found\n", before)
-			os.Exit(1)
-		}
-	}
-	msg := fmt.Sprintf("Registered directive '%s' ", name)
-	if before == "" {
-		msg += "at end of list"
-	} else {
-		msg += fmt.Sprintf("before '%s'", before)
-	}
-	fmt.Printf("[INFO] %s\n", msg)
-}