Button: Make it easier to deactivate certain button types.

Until now the user had to edit `weechat-button-list'.  With this
change there are extra variables used for the default buttons.  The
buttonize? parameter is also `eval'd.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Button: E-Mail buttons.

Default is off.

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