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/auto/regexp.go

@@ -0,0 +1,20 @@
+package auto
+
+// rewriteToExpand rewrites our template string to one that we can give to regexp.ExpandString. This basically
+// involves prefixing any '{' with a '$'.
+func rewriteToExpand(s string) string {
+	// Pretty dumb at the moment, every { will get a $ prefixed.
+	// Also wasteful as we build the string with +=. This is OKish
+	// as we do this during config parsing.
+
+	copy := ""
+
+	for _, c := range s {
+		if c == '{' {
+			copy += "$"
+		}
+		copy += string(c)
+	}
+
+	return copy
+}