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/debug/debug_test.go

@@ -0,0 +1,49 @@
+package debug
+
+import (
+	"io/ioutil"
+	"log"
+	"testing"
+
+	"github.com/coredns/coredns/core/dnsserver"
+
+	"github.com/mholt/caddy"
+)
+
+func TestDebug(t *testing.T) {
+	log.SetOutput(ioutil.Discard)
+
+	tests := []struct {
+		input         string
+		shouldErr     bool
+		expectedDebug bool
+	}{
+		// positive
+		{
+			`debug`, false, true,
+		},
+		// negative
+		{
+			`debug off`, true, false,
+		},
+	}
+
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.input)
+		err := setup(c)
+		cfg := dnsserver.GetConfig(c)
+
+		if test.shouldErr && err == nil {
+			t.Fatalf("Test %d: Expected error but found %s for input %s", i, err, test.input)
+		}
+
+		if err != nil {
+			if !test.shouldErr {
+				t.Fatalf("Test %d: Expected no error but found one for input %s. Error was: %v", i, test.input, err)
+			}
+		}
+		if cfg.Debug != test.expectedDebug {
+			t.Fatalf("Test %d: Expected debug to be: %t, but got: %t, input: %s", i, test.expectedDebug, cfg.Debug, test.input)
+		}
+	}
+}