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/erratic/autopath.go

@@ -0,0 +1,8 @@
+package erratic
+
+import "github.com/coredns/coredns/request"
+
+// AutoPath implements the AutoPathFunc call from the autopath plugin.
+func (e *Erratic) AutoPath(state request.Request) []string {
+	return []string{"a.example.org.", "b.example.org.", ""}
+}