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_test.go

@@ -0,0 +1,25 @@
+package rewrite
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestParse(t *testing.T) {
+	c := caddy.NewTestController("dns", `rewrite`)
+	_, err := rewriteParse(c)
+	if err == nil {
+		t.Errorf("Expected error but found nil for `rewrite`")
+	}
+	c = caddy.NewTestController("dns", `rewrite name`)
+	_, err = rewriteParse(c)
+	if err == nil {
+		t.Errorf("Expected error but found nil for `rewrite name`")
+	}
+	c = caddy.NewTestController("dns", `rewrite name a.com b.com`)
+	_, err = rewriteParse(c)
+	if err != nil {
+		t.Errorf("Expected success but found %s for `rewrite name a.com b.com`", err)
+	}
+}