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/setup.go

@@ -0,0 +1,26 @@
+package loadbalance
+
+import (
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("loadbalance", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	for c.Next() {
+		// TODO(miek): block and option parsing
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		return RoundRobin{Next: next}
+	})
+
+	return nil
+}