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/bind/bind_test.go

@@ -0,0 +1,30 @@
+package bind
+
+import (
+	"testing"
+
+	"github.com/coredns/coredns/core/dnsserver"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetupBind(t *testing.T) {
+	c := caddy.NewTestController("dns", `bind 1.2.3.4`)
+	err := setupBind(c)
+	if err != nil {
+		t.Fatalf("Expected no errors, but got: %v", err)
+	}
+
+	cfg := dnsserver.GetConfig(c)
+	if got, want := cfg.ListenHost, "1.2.3.4"; got != want {
+		t.Errorf("Expected the config's ListenHost to be %s, was %s", want, got)
+	}
+}
+
+func TestBindAddress(t *testing.T) {
+	c := caddy.NewTestController("dns", `bind 1.2.3.bla`)
+	err := setupBind(c)
+	if err == nil {
+		t.Fatalf("Expected errors, but got none")
+	}
+}