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/etcd/msg/path_test.go

@@ -0,0 +1,12 @@
+package msg
+
+import "testing"
+
+func TestPath(t *testing.T) {
+	for _, path := range []string{"mydns", "skydns"} {
+		result := Path("service.staging.skydns.local.", path)
+		if result != "/"+path+"/local/skydns/staging/service" {
+			t.Errorf("Failure to get domain's path with prefix: %s", result)
+		}
+	}
+}