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/loadbalance/README.md

@@ -0,0 +1,22 @@
+# loadbalance
+
+*loadbalance* acts as a round-robin DNS loadbalancer by randomizing the order of A and AAAA records
+ in the answer.
+ 
+ See [Wikipedia](https://en.wikipedia.org/wiki/Round-robin_DNS) about the pros and cons on this
+ setup. It will take care to sort any CNAMEs before any address records, because some stub resolver
+ implementations (like glibc) are particular about that.
+
+## Syntax
+
+~~~
+loadbalance [POLICY]
+~~~
+
+* **POLICY** is how to balance, the default is "round_robin"
+
+## Examples
+
+~~~
+loadbalance round_robin
+~~~