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/trace/README.md

@@ -0,0 +1,73 @@
+# trace
+
+This module enables OpenTracing-based tracing of DNS requests as they go through the
+plugin chain.
+
+## Syntax
+
+The simplest form is just:
+
+~~~
+trace [ENDPOINT-TYPE] [ENDPOINT]
+~~~
+
+* **ENDPOINT-TYPE** is the type of tracing destination. Currently only `zipkin` is supported
+  and that is what it defaults to.
+* **ENDPOINT** is the tracing destination, and defaults to `localhost:9411`. For Zipkin, if
+  ENDPOINT does not begin with `http`, then it will be transformed to `http://ENDPOINT/api/v1/spans`.
+
+With this form, all queries will be traced.
+
+Additional features can be enabled with this syntax:
+
+~~~
+trace [ENDPOINT-TYPE] [ENDPOINT] {
+	every AMOUNT
+	service NAME
+	client_server
+}
+~~~
+
+* `every` **AMOUNT** will only trace one query of each AMOUNT queries. For example, to trace 1 in every
+  100 queries, use AMOUNT of 100. The default is 1.
+* `service` **NAME** allows you to specify the service name reported to the tracing server.
+  Default is `coredns`.
+* `client_server` will enable the `ClientServerSameSpan` OpenTracing feature.
+
+## Zipkin
+You can run Zipkin on a Docker host like this:
+
+```
+docker run -d -p 9411:9411 openzipkin/zipkin
+```
+
+## Examples
+
+Use an alternative Zipkin address:
+
+~~~
+trace tracinghost:9253
+~~~
+
+or
+
+~~~
+trace zipkin tracinghost:9253
+~~~
+
+If for some reason you are using an API reverse proxy or something and need to remap
+the standard Zipkin URL you can do something like:
+
+~~~
+trace http://tracinghost:9411/zipkin/api/v1/spans
+~~~
+
+Trace one query every 10000 queries, rename the service, and enable same span:
+
+~~~
+trace tracinghost:9411 {
+	every 10000
+	service dnsproxy
+	client_server
+}
+~~~