Add inline support for middleware/hosts (#1072)

This fix add inline support for middleware/hosts so that
it is possible to specify hosts file insides the Corefile:
```
hosts inline example.org {
    10.0.0.1 example.org
    fallthrough
}
```

This fix fixes 999.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

test/hosts_file_test.go

@@ -0,0 +1,48 @@
+package test
+
+import (
+	"io/ioutil"
+	"log"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/proxy"
+	"github.com/coredns/coredns/plugin/test"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+func TestHostsInlineLookup(t *testing.T) {
+	corefile := `example.org:0 {
+                       hosts highly_unlikely_to_exist_hosts_file example.org {
+                         10.0.0.1 example.org
+                         fallthrough
+                      }	
+                    }`
+
+	i, udp, _, err := CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer i.Stop()
+
+	log.SetOutput(ioutil.Discard)
+
+	p := proxy.NewLookup([]string{udp})
+	state := request.Request{W: &test.ResponseWriter{}, Req: new(dns.Msg)}
+
+	resp, err := p.Lookup(state, "example.org.", dns.TypeA)
+	if err != nil {
+		t.Fatal("Expected to receive reply, but didn't")
+	}
+	// expect answer section with A record in it
+	if len(resp.Answer) == 0 {
+		t.Fatal("Expected to at least one RR in the answer section, got none")
+	}
+	if resp.Answer[0].Header().Rrtype != dns.TypeA {
+		t.Errorf("Expected RR to A, got: %d", resp.Answer[0].Header().Rrtype)
+	}
+	if resp.Answer[0].(*dns.A).A.String() != "10.0.0.1" {
+		t.Errorf("Expected 10.0.0.1, got: %s", resp.Answer[0].(*dns.A).A.String())
+	}
+}