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/hosts/README.md

@@ -0,0 +1,45 @@
+# hosts
+
+*hosts* enables serving zone data from a `/etc/hosts` style file.
+
+The hosts plugin is useful for serving zones from a /etc/hosts file. It serves from a preloaded
+file that exists on disk. It checks the file for changes and updates the zones accordingly. This
+plugin only supports A, AAAA, and PTR records. The hosts plugin can be used with readily
+available hosts files that block access to advertising servers.
+
+## Syntax
+
+~~~
+hosts [FILE [ZONES...]] {
+    fallthrough
+}
+~~~
+
+* **FILE** the hosts file to read and parse. If the path is relative the path from the *root*
+  directive will be prepended to it. Defaults to /etc/hosts if omitted
+* **ZONES** zones it should be authoritative for. If empty, the zones from the configuration block
+    are used.
+* `fallthrough` If zone matches and no record can be generated, pass request to the next plugin.
+
+## Examples
+
+Load `/etc/hosts` file.
+
+~~~
+hosts
+~~~
+
+Load `example.hosts` file in the current directory.
+
+~~~
+hosts example.hosts
+~~~
+
+Load example.hosts file and only serve example.org and example.net from it and fall through to the
+next plugin if query doesn't match.
+
+~~~
+hosts example.hosts example.org example.net {
+    fallthrough
+}
+~~~