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/file/wildcard.go

@@ -0,0 +1,13 @@
+package file
+
+import "github.com/miekg/dns"
+
+// replaceWithWildcard replaces the left most label with '*'.
+func replaceWithAsteriskLabel(qname string) (wildcard string) {
+	i, shot := dns.NextLabel(qname, 0)
+	if shot {
+		return ""
+	}
+
+	return "*." + qname[i:]
+}