Specify emacs-slack’s entrypoint slack-start

Since currently I’m using a manually checked-out version of slack for
which no autoloads are found, I must manually specify the entry points.
This implies the defer option.

emacs/.emacs.d/init.el

@@ -465,7 +465,7 @@ MODE1 is enabled and vice-versa."
 
 (use-package slack
   :ensure t
-  :defer t
+  :commands (slack-start)
   :load-path "vendor-lisp/emacs-slack"
   :config
   (setq slack-ws-reconnect-auto t))