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/auto/README.md

@@ -0,0 +1,68 @@
+# auto
+
+*auto* enables serving zone data from an RFC 1035-style master file which is automatically picked
+up from disk.
+
+The *auto* plugin is used for an "old-style" DNS server. It serves from a preloaded file that exists
+on disk. If the zone file contains signatures (i.e. is signed, i.e. DNSSEC) correct DNSSEC answers
+are returned. Only NSEC is supported! If you use this setup *you* are responsible for resigning the
+zonefile. New zones or changed zone are automatically picked up from disk.
+
+## Syntax
+
+~~~
+auto [ZONES...] {
+    directory DIR [REGEXP ORIGIN_TEMPLATE [TIMEOUT]]
+    no_reload
+    upstream ADDRESS...
+}
+~~~
+
+**ZONES** zones it should be authoritative for. If empty, the zones from the configuration block
+are used.
+
+* `directory` loads zones from the speficied **DIR**. If a file name matches **REGEXP** it will be
+  used to extract the origin. **ORIGIN_TEMPLATE** will be used as a template for the origin. Strings
+  like `{<number>}` are replaced with the respective matches in the file name, i.e. `{1}` is the
+  first match, `{2}` is the second, etc.. The default is: `db\.(.*)  {1}` e.g. from a file with the
+  name `db.example.com`, the extracted origin will be `example.com`. **TIMEOUT** specifies how often
+  CoreDNS should scan the directory, the default is every 60 seconds. This value is in seconds.
+  The minimum value is 1 second.
+* `no_reload` by default CoreDNS will reload a zone from disk whenever it detects a change to the
+  file. This option disables that behavior.
+* `upstream` defines upstream resolvers to be used resolve external names found (think CNAMEs)
+  pointing to external names. **ADDRESS** can be an IP address, and IP:port or a string pointing to
+  a file that is structured as /etc/resolv.conf.
+
+All directives from the *file* plugin are supported. Note that *auto* will load all zones found,
+even though the directive might only receive queries for a specific zone. I.e:
+
+~~~
+auto example.org {
+    directory /etc/coredns/zones
+}
+~~~
+Will happily pick up a zone for `example.COM`, except it will never be queried, because the *auto*
+directive only is authoritative for `example.ORG`.
+
+## Examples
+
+Load `org` domains from `/etc/coredns/zones/org` and allow transfers to the internet, but send
+notifies to 10.240.1.1
+
+~~~
+auto org {
+    directory /etc/coredns/zones/org
+    transfer to *
+    transfer to 10.240.1.1
+}
+~~~
+
+Load `org` domains from `/etc/coredns/zones/org` and looks for file names as `www.db.example.org`,
+where `example.org` is the origin. Scan every 45 seconds.
+
+~~~
+auto org {
+    directory /etc/coredns/zones/org www\.db\.(.*) {1} 45
+}
+~~~