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/tree/print.go

@@ -0,0 +1,62 @@
+package tree
+
+import "fmt"
+
+// Print prints a Tree. Main use is to aid in debugging.
+func (t *Tree) Print() {
+	if t.Root == nil {
+		fmt.Println("<nil>")
+	}
+	t.Root.print()
+}
+
+func (n *Node) print() {
+	q := newQueue()
+	q.push(n)
+
+	nodesInCurrentLevel := 1
+	nodesInNextLevel := 0
+
+	for !q.empty() {
+		do := q.pop()
+		nodesInCurrentLevel--
+
+		if do != nil {
+			fmt.Print(do.Elem.Name(), " ")
+			q.push(do.Left)
+			q.push(do.Right)
+			nodesInNextLevel += 2
+		}
+		if nodesInCurrentLevel == 0 {
+			fmt.Println()
+		}
+		nodesInCurrentLevel = nodesInNextLevel
+		nodesInNextLevel = 0
+	}
+	fmt.Println()
+}
+
+type queue []*Node
+
+// newQueue returns a new queue.
+func newQueue() queue {
+	q := queue([]*Node{})
+	return q
+}
+
+// push pushes n to the end of the queue.
+func (q *queue) push(n *Node) {
+	*q = append(*q, n)
+}
+
+// pop pops the first element off the queue.
+func (q *queue) pop() *Node {
+	n := (*q)[0]
+	*q = (*q)[1:]
+	return n
+}
+
+// empty returns true when the queue contains zero nodes.
+func (q *queue) empty() bool {
+	return len(*q) == 0
+}