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/health/README.md

@@ -0,0 +1,23 @@
+# health
+
+This module enables a simple health check endpoint. By default it will listen on port 8080.
+
+## Syntax
+
+~~~
+health [ADDRESS]
+~~~
+
+Optionally takes an address; the default is `:8080`. The health path is fixed to `/health`. The
+health endpoint returns a 200 response code and the word "OK" when CoreDNS is healthy. It returns
+a 503. *health* periodically (1s) polls plugin that exports health information. If any of the
+plugin signals that it is unhealthy, the server will go unhealthy too. Each plugin that
+supports health checks has a section "Health" in their README.
+
+## Examples
+
+Run another health endpoint on http://localhost:8091.
+
+~~~
+health localhost:8091
+~~~