Add customization variables to conditionally test
any of the five whitespaces.
(whitespace-spacetab-regexp): Fix doc string.
(whitespace-modes): Add `change-log-mode' to the
list of modes to be checked for bogus whitespaces.
(whitespace-rescan-timer-time): Update documentation.
(whitespace-display-unchecked-whitespaces):
new function to update modeline with untested whitespaces.
(whitespace-buffer): Test only whitespaces whose
checking is turned on, and update modeline using the newly created
`whitespace-display-unchecked-whitespaces'.
(whitespace-cleanup): Cleanup only whitespaces
whose checking is turned on, and update modeline using the newly
created `whitespace-display-unchecked-whitespaces'.
(whitespace-describe): Update documentation.
(whitespace-tickle-timer): Test if `whitespace-rescan-timer-time' is
non-zero before tickling timer.
