Add `ace-jump-search-filter' variable

* ace-jump-mode.el (ace-jump-search-candidate): uses
  `ace-jump-search-filter' if it's not nil

Hi,

This addition might be useful for package writers that use ace-jump-mode.

Here's a use-case for this (jump to open paren but not in comment or string):

    (let ((ace-jump-search-filter (lambda() (not (in-string-or-comment-p)))))
          (ace-jump-char-mode ?\()))