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/proxy/google_test.go

@@ -0,0 +1,5 @@
+package proxy
+
+// TODO(miek):
+// Test cert failures - put those in SERVFAIL messages, but attach error code in TXT
+// Test connecting to a a bad host.