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-draft.el

@@ -402,8 +402,10 @@ fields will be the same as in the original."
 	    ""
 	    mu4e~draft-forward-prefix)
 	  subject))
-      "\n\n"
-      (mu4e~draft-cite-original origmsg))))
+      (unless mu4e-compose-forward-as-attachment
+        (concat
+         "\n\n"
+          (mu4e~draft-cite-original origmsg))))))
 
 (defun mu4e~draft-newmsg-construct ()
   "Create a new message."