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/errors/README.md

@@ -0,0 +1,22 @@
+# errors
+
+*errors* enables error logging.
+
+Any errors encountered during the query processing will be printed to standard output.
+
+## Syntax
+
+~~~
+errors
+~~~
+
+## Examples
+
+Use the *whoami* to respond to queries and Log errors to standard output.
+
+~~~ corefile
+. {
+    whoami
+    errors
+}
+~~~