mu4e: Add mu4e-mark-execute-pre-hook

Add a hook that runs just before executing a mark.

Signed-off-by: Magnus Therning <magnus@therning.org>

mu4e/mu4e.texi

@@ -2114,6 +2114,10 @@ the marks (@key{x}).
 After you have marked some messages, you can execute them with @key{x}
 (@kbd{M-x mu4e-mark-execute-all}).
 
+A hook, @code{mu4e-mark-execute-pre-hook}, is available which is run
+right before execution of each mark. The hook is called with two
+arguments, the mark and the message itself.
+
 @node Leaving the headers buffer
 @section Leaving the headers buffer