Add support for external_id in topic creation (#263)

* Add support for external_id in topic creation
* add get_topic_url_by_external_id

Author: thomasrossetto

lib/discourse_api/api/topics.rb

@@ -11,7 +11,15 @@ def create_topic(args = {})
           API
             .params(args)
             .required(:title, :raw)
-            .optional(:skip_validations, :category, :auto_track, :created_at, :api_username, :tags)
+            .optional(
+              :skip_validations,
+              :category,
+              :auto_track,
+              :created_at,
+              :api_username,
+              :tags,
+              :external_id,
+            )
         post("/posts", args.to_h)
       end
 
@@ -111,6 +119,10 @@ def bookmark_topic(topic_id)
       def remove_topic_bookmark(topic_id)
         put("/t/#{topic_id}/remove_bookmarks.json")
       end
+
+      def get_topic_url_by_external_id(external_id)
+        get("/t/external_id/#{external_id}")
+      end
     end
   end
 end