* wl-vars.el (wl-message-use-header-narrowing): New user option.

* wl-vars.el (wl-message-use-header-narrowing): New user option.
(wl-message-header-narrowing-fields): Ditto.
(wl-message-header-narrowing-lines): Ditto.
(wl-message-header-narrowing-string): Ditto.

* wl-summary.el (wl-summary-mode-map): Bind
wl-summary-toggle-header-narrowing to "C-cC-f".
(wl-summary-toggle-header-narrowing): New function.

* wl-message.el (wl-message-buffer-create): Call
wl-message-header-narrowing-setup.
(wl-message-redisplay): Call wl-message-header-narrowing.
(wl-message-header-narrowing): New function.
(wl-message-header-narrowing-map): New keymap.
(wl-message-header-narrowing-widen-map): New keymap.
(wl-message-header-narrowing-again-at-mouse): New function.
(wl-message-header-narrowing-1): Ditto.
(wl-message-header-narrowing-widen-at-mouse): Ditto.
(wl-message-header-narrowing-setup): Ditto.
(wl-message-header-narrowing-toggle): Ditto.

* wl-highlight.el (wl-message-header-narrowing-face): New face.