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/rcode/rcode_test.go

@@ -0,0 +1,29 @@
+package rcode
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestToString(t *testing.T) {
+	tests := []struct {
+		in       int
+		expected string
+	}{
+		{
+			dns.RcodeSuccess,
+			"NOERROR",
+		},
+		{
+			28,
+			"RCODE28",
+		},
+	}
+	for i, test := range tests {
+		got := ToString(test.in)
+		if got != test.expected {
+			t.Errorf("Test %d, expected %s, got %s", i, test.expected, got)
+		}
+	}
+}