Support 'User (mention)' style mentions.

disco/bot/bot.py

@@ -312,6 +312,7 @@ class Bot(LoggingClass):
                         if member.nick:
                             content = content.replace(member.mention, '', 1)
                         content = content.replace(member.user.mention, '', 1)
+                        content = content.replace(member.user.mention_nickname, '', 1)
                 else:
                     content = content.replace(self.client.state.me.mention, '', 1)
             elif mention_everyone:

disco/types/user.py

@@ -44,6 +44,10 @@ class User(SlottedModel, with_equality('id'), with_hash('id')):
     def mention(self):
         return '<@{}>'.format(self.id)
 
+    @property
+    def mention_nickname(self):
+        return '<@!{}>'.format(self.id)
+
     def open_dm(self):
         return self.client.api.users_me_dms_create(self.id)