Generic filtering callback support for popup-isearch w/example filter by docstring

I've found the docstring search case useful for searching for function argument types, keywords like deprecated, async, callback, and even elisp defun keywords like &rest and &key.

I thought that rather than just add docstring, generic filtering functionality should be exposed.

I've submitted this pull request as a potential fix for issue #39

Example use of this patch w/auto-complete:

```
(defun ac-isearch-doc ()
  (interactive)
  (when (ac-menu-live-p)
    (ac-cancel-show-menu-timer)
    (ac-show-menu)
    (if ac-use-quick-help
        (let ((popup-menu-show-quick-help-function
               (if (ac-quick-help-use-pos-tip-p)
                   'ac-pos-tip-show-quick-help
                 'popup-menu-show-quick-help)))
          (popup-isearch ac-menu
                         :callback 'ac-isearch-callback
                         :help-delay ac-quick-help-delay
			 :filter 'popup-isearch-filter-list-by-doc))
      (popup-isearch ac-menu 
		     :callback 'ac-isearch-callback
		     :filter 'popup-isearch-filter-list-by-doc))))
```