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/whoami/setup_test.go

@@ -0,0 +1,19 @@
+package whoami
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetupWhoami(t *testing.T) {
+	c := caddy.NewTestController("dns", `whoami`)
+	if err := setupWhoami(c); err != nil {
+		t.Fatalf("Expected no errors, but got: %v", err)
+	}
+
+	c = caddy.NewTestController("dns", `whoami example.org`)
+	if err := setupWhoami(c); err == nil {
+		t.Fatalf("Expected errors, but got: %v", err)
+	}
+}