Speed up annotations by only annotating visible candidates

I was previously annotating all candidates, which is pointless. With
this change only the visible candidates will be annotated.

(I have to say, I'm a little surprised annotating all candidates was
so slow for some people (see #18): it seemed fast enough on my cheap 6
year-old chromebook, running Emacs in a Linux VM! I may need to get an
even slower computer...)

icomplete-vertical.el

@@ -6,7 +6,7 @@
 ;; Keywords: convenience, completion
 ;; Version: 0.3
 ;; Homepage: https://github.com/oantolin/icomplete-vertical
-;; Package-Requires: ((emacs "24.4"))
+;; Package-Requires: ((emacs "25.1"))
 
 ;; This program is free software; you can redistribute it and/or modify
 ;; it under the terms of the GNU General Public License as published by
@@ -212,7 +212,7 @@ To be used as filter return advice for `icomplete--sorted-completions'."
                        'face 'completions-annotations
                        annotation))
                     (concat candidate annotation)))
-                completions)))
+                (seq-take completions icomplete-vertical-prospects-height))))
           (setcdr last save)             ; restore
           (setcdr (last annotated) save) ; imitate
           annotated)))))