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/file/README.md

@@ -0,0 +1,55 @@
+# file
+
+*file* enables serving zone data from an RFC 1035-style master file.
+
+The file 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.
+
+## Syntax
+
+~~~
+file DBFILE [ZONES...]
+~~~
+
+* **DBFILE** the database file to read and parse. If the path is relative the path from the *root*
+  directive will be prepended to it.
+* **ZONES** zones it should be authoritative for. If empty, the zones from the configuration block
+    are used.
+
+If you want to round robin A and AAAA responses look at the *loadbalance* plugin.
+
+TSIG key configuration is TODO; directive format for transfer will probably be extended with
+TSIG key information, something like `transfer out [ADDRESS...] key [NAME[:ALG]] [BASE64]`
+
+~~~
+file DBFILE [ZONES... ] {
+    transfer to ADDRESS...
+    no_reload
+    upstream ADDRESS...
+}
+~~~
+
+* `transfer` enables zone transfers. It may be specified multiples times. `To` or `from` signals
+  the direction. **ADDRESS** must be denoted in CIDR notation (127.0.0.1/32 etc.) or just as plain
+  addresses. The special wildcard `*` means: the entire internet (only valid for 'transfer to').
+  When an address is specified a notify message will be send whenever the zone is reloaded.
+* `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. This is only really useful when CoreDNS is configured as a proxy, for
+  normal authoritative serving you don't need *or* want to use this. **ADDRESS** can be an IP
+  address, and IP:port or a string pointing to a file that is structured as /etc/resolv.conf.
+
+## Examples
+
+Load the `example.org` zone from `example.org.signed` and allow transfers to the internet, but send
+notifies to 10.240.1.1
+
+~~~
+file example.org.signed example.org {
+    transfer to *
+    transfer to 10.240.1.1
+}
+~~~