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/file/zone_test.go

@@ -0,0 +1,30 @@
+package file
+
+import "testing"
+
+func TestNameFromRight(t *testing.T) {
+	z := NewZone("example.org.", "stdin")
+
+	tests := []struct {
+		in       string
+		labels   int
+		shot     bool
+		expected string
+	}{
+		{"example.org.", 0, false, "example.org."},
+		{"a.example.org.", 0, false, "example.org."},
+		{"a.example.org.", 1, false, "a.example.org."},
+		{"a.example.org.", 2, true, "a.example.org."},
+		{"a.b.example.org.", 2, false, "a.b.example.org."},
+	}
+
+	for i, tc := range tests {
+		got, shot := z.nameFromRight(tc.in, tc.labels)
+		if got != tc.expected {
+			t.Errorf("Test %d: expected %s, got %s\n", i, tc.expected, got)
+		}
+		if shot != tc.shot {
+			t.Errorf("Test %d: expected shot to be %t, got %t\n", i, tc.shot, shot)
+		}
+	}
+}