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.cfg

@@ -0,0 +1,51 @@
+# Directives are registered in the order they should be
+# executed.
+#
+# Ordering is VERY important. Every plugin will
+# feel the effects of all other plugin below
+# (after) them during a request, but they must not
+# care what plugin above them are doing.
+
+# How to rebuild with updated plugin configurations:
+# Modify the list below and run `go gen && go build`
+
+# The parser takes the input format of
+#     <order>:<plugin-name>:<package-name>
+# Or
+#     <order>:<plugin-name>:<fully-qualified-package-name>
+#
+# External plugin example:
+# 80:log:github.com/coredns/coredns/plugin/log
+# Local plugin example:
+# 80:log:log
+
+1:tls:tls
+10:root:root
+20:bind:bind
+30:debug:debug
+40:trace:trace
+50:health:health
+60:pprof:pprof
+70:prometheus:metrics
+80:errors:errors
+90:log:log
+100:autopath:autopath
+110:dnstap:dnstap
+120:chaos:chaos
+130:cache:cache
+140:rewrite:rewrite
+150:loadbalance:loadbalance
+160:dnssec:dnssec
+170:reverse:reverse
+180:hosts:hosts
+190:federation:federation
+200:kubernetes:kubernetes
+210:file:file
+220:auto:auto
+230:secondary:secondary
+240:etcd:etcd
+250:proxy:proxy
+260:erratic:erratic
+270:whoami:whoami
+500:startup:github.com/mholt/caddy/startupshutdown
+510:shutdown:github.com/mholt/caddy/startupshutdown