Add a 'big-indent style to `whitespace-mode'.

Add a 'big-indent style to `whitespace-mode'.

* lisp/whitespace.el (whitespace-style, whitespace-big-indent)
(whitespace-big-indent-regexp, whitespace-style-value-list)
(whitespace-toggle-option-alist, whitespace-interactive-char)
(whitespace-toggle-options)
(global-whitespace-toggle-options, whitespace-help-text)
(whitespace-style-face-p, whitespace-color-on): Add a 'big-indent
style to `whitespace-mode' to indicate that the line indentation
is too deep.  By default, 32 SPACEs or four TABs are considered
too many but `whitespace-big-indent-regexp' can be configured.