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/pkg/trace/trace.go

@@ -0,0 +1,12 @@
+package trace
+
+import (
+	"github.com/coredns/coredns/plugin"
+	ot "github.com/opentracing/opentracing-go"
+)
+
+// Trace holds the tracer and endpoint info
+type Trace interface {
+	plugin.Handler
+	Tracer() ot.Tracer
+}