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/pkg/rcode/rcode.go

@@ -0,0 +1,16 @@
+package rcode
+
+import (
+	"strconv"
+
+	"github.com/miekg/dns"
+)
+
+// ToString convert the rcode to the official DNS string, or to "RCODE"+value if the RCODE
+// value is unknown.
+func ToString(rcode int) string {
+	if str, ok := dns.RcodeToString[rcode]; ok {
+		return str
+	}
+	return "RCODE" + strconv.Itoa(rcode)
+}