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/closest.go

@@ -0,0 +1,24 @@
+package file
+
+import (
+	"github.com/coredns/coredns/plugin/file/tree"
+
+	"github.com/miekg/dns"
+)
+
+// ClosestEncloser returns the closest encloser for qname.
+func (z *Zone) ClosestEncloser(qname string) (*tree.Elem, bool) {
+
+	offset, end := dns.NextLabel(qname, 0)
+	for !end {
+		elem, _ := z.Tree.Search(qname)
+		if elem != nil {
+			return elem, true
+		}
+		qname = qname[offset:]
+
+		offset, end = dns.NextLabel(qname, offset)
+	}
+
+	return z.Tree.Search(z.origin)
+}