plugin/pkg/log: add plugin logging (#1716)

Add per plugin logging to make it explicit what is logging, if you
include this package under the name clog (coredns log), you can do the
following:

log := clog.NewWithPlugin{whoami{}} // e.g.

And then just log.Info(...); these will then include the plugin ala:
[INFO] plugin/whoami: stuff

So we only need to init the logger and then just use it.

plugin/pkg/log/plugin_test.go

@@ -0,0 +1,32 @@
+package log
+
+import (
+	"bytes"
+	"context"
+	golog "log"
+	"strings"
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+type p struct{}
+
+func (p p) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	return 0, nil
+}
+
+func (p p) Name() string { return "testplugin" }
+
+func TestPlugins(t *testing.T) {
+	var f bytes.Buffer
+	const ts = "test"
+	golog.SetOutput(&f)
+
+	lg := NewWithPlugin(p{})
+
+	lg.Info(ts)
+	if x := f.String(); !strings.Contains(x, "plugin/testplugin") {
+		t.Errorf("Expected log to be %s, got %s", info+ts, x)
+	}
+}