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/loadbalance/handler.go

@@ -0,0 +1,23 @@
+// Package loadbalance is plugin for rewriting responses to do "load balancing"
+package loadbalance
+
+import (
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+// RoundRobin is plugin to rewrite responses for "load balancing".
+type RoundRobin struct {
+	Next plugin.Handler
+}
+
+// ServeDNS implements the plugin.Handler interface.
+func (rr RoundRobin) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	wrr := &RoundRobinResponseWriter{w}
+	return plugin.NextOrFailure(rr.Name(), rr.Next, ctx, wrr, r)
+}
+
+// Name implements the Handler interface.
+func (rr RoundRobin) Name() string { return "loadbalance" }