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/secondary/secondary.go

@@ -0,0 +1,10 @@
+// Package secondary implements a secondary plugin.
+package secondary
+
+import "github.com/coredns/coredns/plugin/file"
+
+// Secondary implements a secondary plugin that allows CoreDNS to retrieve (via AXFR)
+// zone information from a primary server.
+type Secondary struct {
+	file.File
+}