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/tls/tls.go

@@ -0,0 +1,37 @@
+package tls
+
+import (
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/tls"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("tls", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	config := dnsserver.GetConfig(c)
+
+	if config.TLSConfig != nil {
+		return plugin.Error("tls", c.Errf("TLS already configured for this server instance"))
+	}
+
+	for c.Next() {
+		args := c.RemainingArgs()
+		if len(args) != 3 {
+			return plugin.Error("tls", c.ArgErr())
+		}
+		tls, err := tls.NewTLSConfig(args[0], args[1], args[2])
+		if err != nil {
+			return plugin.Error("tls", err)
+		}
+		config.TLSConfig = tls
+	}
+	return nil
+}