Enable flycheck for Emacs Lisp by default

It used to be slow, but it's not the case anymore.  This should help identifying
problems in my ELisp code, but might cause some trouble on Windows.  Let's see …

init.el

@@ -2564,7 +2564,9 @@ With given ARG, display files in `db/important-document-path’."
 ;; Lisp Dialects
 
 (use-package elisp-mode
-  :config (add-hook 'emacs-lisp-mode-hook 'turn-on-lispy-when-available))
+  :config (progn
+            (add-hook 'emacs-lisp-mode-hook 'turn-on-lispy-when-available)
+            (add-hook 'emacs-lisp-mode-hook 'flycheck-mode)))
 
 (use-package geiser
   :commands (geiser-mode))