Add support for XEP-0382: Spoiler Messages

Fixes SMACK-795.

documentation/extensions/spoiler.md

@@ -0,0 +1,33 @@
+Spoiler Messages
+================
+
+[Back](index.md)
+
+Spoiler Messages can be used to indicate that the body of a message is a spoiler and should be displayed as such.
+
+## Usage
+
+To get an instance of the SpoilerManager, call 
+```
+SpoilerManager manager = SpoilerManager.getInstanceFor(connection);
+```
+This will automatically add Spoilers to the list of supported features of your client.
+
+The manager can then be used to add SpoilerElements to messages like follows:
+```
+Message message = new Message();
+
+// spoiler without hint
+SpoilerElement.addSpoiler(message);
+
+// spoiler with hint about content
+SpoilerElement.addSpoiler(message, "End of Love Story");
+
+// spoiler with localized hint
+SpoilerElement.addSpoiler(message, "de", "Der Kuchen ist eine Lüge");
+```
+
+To get Spoilers from a message call
+```
+Map<String, String> spoilers = SpoilerElement.getSpoilers(message);
+```