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/debug/README.md

@@ -0,0 +1,20 @@
+# debug
+
+*debug* disables the automatic recovery upon a CoreDNS crash so that you'll get a nice stack trace.
+
+Note that the *errors* plugin (if loaded) will also set a `recover` negating this setting.
+The main use of *debug* is to help testing.
+
+## Syntax
+
+~~~ txt
+debug
+~~~
+
+## Examples
+
+Disable CoreDNS' ability to recover from crashes:
+
+~~~ txt
+debug
+~~~