Add support for notifying with Slack attachments. (#2914)

* Add support for notifying with Slack messages.

When creating notifications, this allows you to pass in `attachments`
with the `data`. It's an array of attachments as defined in
https://api.slack.com/docs/message-attachments

When passing in attachments, message is still required, but it's okay to
be a blank string.

* Split over multiple lines

* Make sure attachments gets assigned, even if there isn't attachment data

homeassistant/components/notify/slack.py

@@ -51,7 +51,15 @@ class SlackNotificationService(BaseNotificationService):
         import slacker
 
         channel = kwargs.get('target') or self._default_channel
+        data = kwargs.get('data')
+        if data:
+            attachments = data.get('attachments')
+        else:
+            attachments = None
+
         try:
-            self.slack.chat.post_message(channel, message, as_user=True)
+            self.slack.chat.post_message(channel, message,
+                                         as_user=True,
+                                         attachments=attachments)
         except slacker.Error:
             _LOGGER.exception("Could not send slack notification")