Close #639: Add option to control if we replace SLIME on load

Use it to control whether to prompt interactively about replacing
SLIME, and if not prompting whether to replace or not replace.  Thanks
to Stefan Monnier for suggesting improvements to the original patch.

* sly.el (sly-replace-slime): New defcustom.

Co-authored-by João Távora <joaotavora@gmail.com>

sly.el

@@ -7475,11 +7475,25 @@ can be found."
 ;;;###autoload
 (add-hook 'lisp-mode-hook 'sly-editing-mode)
 
+(defcustom sly-replace-slime 'ask
+  "Specify whether SLY should replace SLIME at load time.
+
+This only has an effect if parts of SLIME components are already
+loaded (e.g. in `lisp-mode-hook').
+
+If `ask' prompt the user at load-time; if nil never replace; if t or
+other non-nil value to unconditionally replace SLIME."
+  :type '(choice (const :tag "Ask user" ask)
+                 (const :tag "Do not replace SLIME" nil)
+                 (const :tag "Do replace SLIME" t)))
+
 (cond
  ((or (not (memq 'slime-lisp-mode-hook lisp-mode-hook))
       noninteractive
       (prog1
-          (y-or-n-p "[sly] SLIME detected in `lisp-mode-hook', causes keybinding conflicts.  Remove it for this Emacs session?")
+          (if (eq sly-replace-slime 'ask)
+              (y-or-n-p "[sly] SLIME detected in `lisp-mode-hook', causes keybinding conflicts.  Remove it for this Emacs session?")
+            sly-replace-slime)
         (warn "To restore SLIME in this session, customize `lisp-mode-hook'
 and replace `sly-editing-mode' with `slime-lisp-mode-hook'.")))
   (remove-hook 'lisp-mode-hook 'slime-lisp-mode-hook)