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/proxy/setup.go

@@ -0,0 +1,46 @@
+package proxy
+
+import (
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("proxy", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	upstreams, err := NewStaticUpstreams(&c.Dispenser)
+	if err != nil {
+		return plugin.Error("proxy", err)
+	}
+
+	t := dnsserver.GetConfig(c).Handler("trace")
+	P := &Proxy{Trace: t}
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		P.Next = next
+		P.Upstreams = &upstreams
+		return P
+	})
+
+	c.OnStartup(OnStartupMetrics)
+
+	for i := range upstreams {
+		u := upstreams[i]
+		c.OnStartup(func() error {
+			return u.Exchanger().OnStartup(P)
+		})
+		c.OnShutdown(func() error {
+			return u.Exchanger().OnShutdown(P)
+		})
+		// Register shutdown handlers.
+		c.OnShutdown(u.Stop)
+	}
+
+	return nil
+}