bug#20351: isearch-lax-whitespace should be customizable variable

[Juri Linkov]

> M-x customize-variable
> does NOT list isearch-lax-whitespace as a customizable variable

I believe both ‘isearch-lax-whitespace’ and ‘character-fold-search’
should be customizable.  bug#11378 proposed a patch that adds a new
user variable to customize all such search parameters:

(defcustom isearch-keep-mode-variables nil
  "A set of search variables to keep between different searches.
When a search variable is customized to exist in this set, then
starting a new search doesn't reset the corresponding isearch variable
to its default value, thus keeping the value from the previous search
\(changed using toggling commands)."
  :type '(set (const :tag "Case folding" isearch-case-fold-search)
              (const :tag "Character folding" character-fold-search)
              (const :tag "Invisible text" isearch-invisible)
              (const :tag "Filters" isearch-filter-predicate)
              (const :tag "Lax whitespace" isearch-lax-whitespace)
              (const :tag "Regexp lax whitespace" 
isearch-regexp-lax-whitespace))
  :group 'isearch)