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/file_test.go

@@ -0,0 +1,31 @@
+package file
+
+import (
+	"strings"
+	"testing"
+)
+
+func BenchmarkFileParseInsert(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Parse(strings.NewReader(dbMiekENTNL), testzone, "stdin", 0)
+	}
+}
+
+func TestParseNoSOA(t *testing.T) {
+	_, err := Parse(strings.NewReader(dbNoSOA), "example.org.", "stdin", 0)
+	if err == nil {
+		t.Fatalf("zone %q should have failed to load", "example.org.")
+	}
+	if !strings.Contains(err.Error(), "no SOA record") {
+		t.Fatalf("zone %q should have failed to load with no soa error: %s", "example.org.", err)
+	}
+}
+
+const dbNoSOA = `
+$TTL         1M
+$ORIGIN      example.org.
+
+www          IN  A      192.168.0.14
+mail         IN  A      192.168.0.15
+imap         IN  CNAME  mail
+`