Add Slack support

vendor/github.com/nlopes/slack/messageID.go

@@ -0,0 +1,30 @@
+package slack
+
+import "sync"
+
+// IDGenerator provides an interface for generating integer ID values.
+type IDGenerator interface {
+	Next() int
+}
+
+// NewSafeID returns a new instance of an IDGenerator which is safe for
+// concurrent use by multiple goroutines.
+func NewSafeID(startID int) IDGenerator {
+	return &safeID{
+		nextID: startID,
+		mutex:  &sync.Mutex{},
+	}
+}
+
+type safeID struct {
+	nextID int
+	mutex  *sync.Mutex
+}
+
+func (s *safeID) Next() int {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	id := s.nextID
+	s.nextID++
+	return id
+}