Consider adding a :skip selector to use existing Org Agenda skip predicates

I use (org-review)[https://github.com/brabalan/org-review] to remind me to regularly revisit projects to make sure they're still relevant, look for upcoming tasks or work that needs to be rescheduled.

I'd really like to create a super-agenda group to show me the things that are up for review, but I can't figure out how. As a result, I have a lot of blocks like this in `org-agenda-custom-commands`:

```lisp
("As" "All self tasks"
((tags-todo "-CANCELLED+a@self"
((org-agenda-overriding-header "Scheduled for review")
(org-agenda-tag-filter-preset
'("+a@self"))
(org-agenda-skip-function 'org-review-agenda-skip)
(org-agenda-cmp-user-defined 'org-review-compare)
(org-super-agenda-groups nil)
(org-agenda-sorting-strategy
'(user-defined-down))))
(tags-todo "a@self" nil))
```

and the Agenda view I use most often is similar:

``` ("O" "super-agenda fOcus"
((tags-todo "-CANCELLED/"
((org-agenda-overriding-header "Scheduled for review")
(org-agenda-skip-function 'org-review-agenda-skip)
(org-agenda-cmp-user-defined 'org-review-compare)
(org-super-agenda-groups nil)
(org-agenda-sorting-strategy
'(user-defined-down))))
(agenda ""
( ..... ))
nil nil)
```

The new `:pred` doesn't quite help me here; org-review does provide (`org-review-toreview-p` )[https://github.com/brabalan/org-review/blob/master/org-review.el#L160] which comes close, doesn't quite make it.

I'm not quite sure where to go from here. It feels like adding support for `:skip` which takes skip functions of the same sort as those used for `org-agenda-skip-function` might help with this and also with other things which rely on a skip function; but I'm not sure of how slow that would be to execute.

Alternatively, perhas it would be simpler to add to `org-review` a predicate function that would work with the existing `:pred`