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/backend.go

@@ -0,0 +1,32 @@
+package plugin
+
+import (
+	"github.com/coredns/coredns/plugin/etcd/msg"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// ServiceBackend defines a (dynamic) backend that returns a slice of service definitions.
+type ServiceBackend interface {
+	// Services communicates with the backend to retrieve the service definition. Exact indicates
+	// on exact much are that we are allowed to recurs.
+	Services(state request.Request, exact bool, opt Options) ([]msg.Service, error)
+
+	// Reverse communicates with the backend to retrieve service definition based on a IP address
+	// instead of a name. I.e. a reverse DNS lookup.
+	Reverse(state request.Request, exact bool, opt Options) ([]msg.Service, error)
+
+	// Lookup is used to find records else where.
+	Lookup(state request.Request, name string, typ uint16) (*dns.Msg, error)
+
+	// Returns _all_ services that matches a certain name.
+	// Note: it does not implement a specific service.
+	Records(state request.Request, exact bool) ([]msg.Service, error)
+
+	// IsNameError return true if err indicated a record not found condition
+	IsNameError(err error) bool
+}
+
+// Options are extra options that can be specified for a lookup.
+type Options struct{}