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/rewrite/setup.go

@@ -0,0 +1,42 @@
+package rewrite
+
+import (
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("rewrite", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	rewrites, err := rewriteParse(c)
+	if err != nil {
+		return plugin.Error("rewrite", err)
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		return Rewrite{Next: next, Rules: rewrites}
+	})
+
+	return nil
+}
+
+func rewriteParse(c *caddy.Controller) ([]Rule, error) {
+	var rules []Rule
+
+	for c.Next() {
+		args := c.RemainingArgs()
+		rule, err := newRule(args...)
+		if err != nil {
+			return nil, err
+		}
+		rules = append(rules, rule)
+	}
+	return rules, nil
+}