Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/pkg/nonwriter/nonwriter_test.go

@@ -0,0 +1,19 @@
+package nonwriter
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestNonWriter(t *testing.T) {
+	nw := New(nil)
+	m := new(dns.Msg)
+	m.SetQuestion("example.org.", dns.TypeA)
+	if err := nw.WriteMsg(m); err != nil {
+		t.Errorf("Got error when writing to nonwriter: %s", err)
+	}
+	if x := nw.Msg.Question[0].Name; x != "example.org." {
+		t.Errorf("Expacted 'example.org.' got %q:", x)
+	}
+}