mu4e: Add possibility to forward messages as attachments

This commit adds a global variable
mu4e-compose-forward-as-attachment. To enable choosing forwarding
method on a per-message basis would probably require either:

• changing the mu server backend so that it distinguishes between
  inline forwarding and forwarding as attachment;
• changing the mu server backend so that it doesn’t return attachments
  at all and making both inline and as attachment forwarding via
  MIME (and also making mu4e actually display MIME-enclosed inline
  emails).

mu4e/mu4e-vars.el

@@ -393,6 +393,10 @@ the From: address.)"
   :type 'string
   :group 'mu4e-compose)
 
+(defcustom mu4e-compose-forward-as-attachment nil
+  "Whether to forward messages as attachments instead of inline."
+  :type 'boolean
+  :group 'mu4e-compose)
 
 ;; backward compatibility
 (make-obsolete-variable 'mu4e-reply-to-address 'mu4e-compose-reply-to-address