Allow sending direct messages with slack module

lib/ansible/modules/extras/notification/slack.py

@@ -141,7 +141,10 @@ def build_payload_for_slack(module, text, channel, username, icon_url, icon_emoj
     else:
         payload = dict(attachments=[dict(text=text, color=color)])
     if channel is not None:
-        payload['channel'] = channel if (channel[0] == '#') else '#'+channel
+        if (channel[0] == '#') or (channel[0] == '@')
+            payload['channel'] = channel
+        else 
+            payload['channel'] = '#'+channel
     if username is not None:
         payload['username'] = username
     if icon_emoji is not None: