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/auto/regexp_test.go

@@ -0,0 +1,20 @@
+package auto
+
+import "testing"
+
+func TestRewriteToExpand(t *testing.T) {
+	tests := []struct {
+		in       string
+		expected string
+	}{
+		{in: "", expected: ""},
+		{in: "{1}", expected: "${1}"},
+		{in: "{1", expected: "${1"},
+	}
+	for i, tc := range tests {
+		got := rewriteToExpand(tc.in)
+		if got != tc.expected {
+			t.Errorf("Test %d: Expected error %v, but got %v", i, tc.expected, got)
+		}
+	}
+}