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/test/file_test.go

@@ -0,0 +1,11 @@
+package test
+
+import "testing"
+
+func TestTempFile(t *testing.T) {
+	_, f, e := TempFile(".", "test")
+	if e != nil {
+		t.Fatalf("failed to create temp file: %s", e)
+	}
+	defer f()
+}