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/dnsutil/join_test.go

@@ -0,0 +1,20 @@
+package dnsutil
+
+import "testing"
+
+func TestJoin(t *testing.T) {
+	tests := []struct {
+		in  []string
+		out string
+	}{
+		{[]string{"bla", "bliep", "example", "org"}, "bla.bliep.example.org."},
+		{[]string{"example", "."}, "example."},
+		{[]string{"."}, "."},
+	}
+
+	for i, tc := range tests {
+		if x := Join(tc.in); x != tc.out {
+			t.Errorf("Test %d, expected %s, got %s", i, tc.out, x)
+		}
+	}
+}