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/whoami/setup.go

@@ -0,0 +1,28 @@
+package whoami
+
+import (
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"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 plugin.Error("whoami", c.ArgErr())
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		return Whoami{}
+	})
+
+	return nil
+}