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

[Juri Linkov]

>> (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)
>
> Though this is not strictly in conflict with turning the individual
> variables into defcustoms, I think it's a bit in ideological conflict, and
> we should probably pick one of the two.

Would it be better to pollute the namespace with N*M more customizable variables
like ‘isearch-keep-case-fold-search’, ‘isearch-keep-lax-whitespace’,
‘isearch-initial-case-fold-search’, ‘isearch-initial-lax-whitespace’, I'm not 
sure.