Add ReplyUsers to Msg Struct

This is something that Slack returns by default on API response.

messages.go

@@ -101,6 +101,7 @@ type Msg struct {
 
 	// channels.replies, groups.replies, im.replies, mpim.replies
 	ReplyCount   int     `json:"reply_count,omitempty"`
+	ReplyUsers   []string `json:"reply_users,omitempty"`
 	Replies      []Reply `json:"replies,omitempty"`
 	ParentUserId string  `json:"parent_user_id,omitempty"`
 	LatestReply  string  `json:"latest_reply,omitempty"`