Use `unless' instead of `when'/`not'.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Added customize support for weechat.el.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Added support for color codes.

The function `weechat-handle-color-codes' converts Weechat color codes
into propertized Emacs' strings.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>