Rewrite edns0 (#561)

* Add edns0 code rewrite

* check arg count

* change `new`; set EDNS0 if request doesn't have it set

* change set to replace_or_append

* change to append_or_replace

* return error in new

* update documents

* fixt UT

* return error

* go fmt

* Rework for more general EDNS0 use

Also changed how rules are created and validated. Implements
EDNS0 NSID in addition to local.

* go fmt

* README updates, NSID tests and fixes

* gofmt -s -w

* Fix tests for rewrite syntax change

* Add tests, fix error message

* Review nits

* Missed on nit

* More tests, integration test, fix edns0 parse issue

* Fix README, use RewriteIgnored

* go fmt

middleware/rewrite/setup_test.go

@@ -0,0 +1,25 @@
+package rewrite
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestParse(t *testing.T) {
+	c := caddy.NewTestController("dns", `rewrite`)
+	_, err := rewriteParse(c)
+	if err == nil {
+		t.Errorf("Expected error but found nil for `rewrite`")
+	}
+	c = caddy.NewTestController("dns", `rewrite name`)
+	_, err = rewriteParse(c)
+	if err == nil {
+		t.Errorf("Expected error but found nil for `rewrite name`")
+	}
+	c = caddy.NewTestController("dns", `rewrite name a.com b.com`)
+	_, err = rewriteParse(c)
+	if err != nil {
+		t.Errorf("Expected success but found %s for `rewrite name a.com b.com`", err)
+	}
+}