Allow the user to specify a custom function to build the regexp in isearch

[Konubinix]

If the user likes to use helm or ivy style of matching candidates, they can provide their own regexp builer.

For instance, in case you like to 'foo bar' to match 'foo.+bar', you can provide a function that replaces spaces by '.+'.

For instance, I set popup-isearch-regexp-builder-function to ivy--regex-plus. (see https://github.com/abo-abo/swiper/blob/master/ivy.el#L1418)

[syohex]

Please use defcustom instead of defvar for such variable.

[Konubinix]

Done :-)

[syohex]

Thanks