middleware/whoami: add (#264)

Add a new middleware that tells you who you are; IP, port and transport
is echoed back.

Also some various cleanup and documentation improvements while at it:

* ResponseWriter: improve the documentation of these helper functions.
* And add an NextHandler for use in tests. Make chaos_test.go and
* whoam_test.go use it.

middleware/whoami/setup.go

@@ -0,0 +1,28 @@
+package whoami
+
+import (
+	"github.com/miekg/coredns/core/dnsserver"
+	"github.com/miekg/coredns/middleware"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("whoami", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setupWhoami,
+	})
+}
+
+func setupWhoami(c *caddy.Controller) error {
+	c.Next() // 'whoami'
+	if c.NextArg() {
+		return middleware.Error("whoami", c.ArgErr())
+	}
+
+	dnsserver.GetConfig(c).AddMiddleware(func(next dnsserver.Handler) dnsserver.Handler {
+		return Whoami{Next: next}
+	})
+
+	return nil
+}