wl-draft-config-alist support 'forward' key
Like as 'reply' key, when you invoke wl-summary-forward, you can
perform matching in original message buffer.
* doc/wl-ja.texi (Dynamical Message Re-arrangement): Describe it.
* doc/wl.texi (Dynamical Message Re-arrangement): Likewise.
* wl/wl-draft.el (wl-draft-forward-buffer): New buffer local
variable.
(wl-draft-forward): Set wl-draft-forward-buffer to original
buffer.
(wl-draft-config-exec): Support 'forward' key.
* wl/wl-vars.el (wl-draft-config-alist): Update docstring.