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/bind/setup.go

@@ -0,0 +1,24 @@
+package bind
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func setupBind(c *caddy.Controller) error {
+	config := dnsserver.GetConfig(c)
+	for c.Next() {
+		if !c.Args(&config.ListenHost) {
+			return plugin.Error("bind", c.ArgErr())
+		}
+	}
+	if net.ParseIP(config.ListenHost) == nil {
+		return plugin.Error("bind", fmt.Errorf("not a valid IP address: %s", config.ListenHost))
+	}
+	return nil
+}