Add option to fill window with stories

If hackernews-items-per-page is nil, the number of stories is three
less than the window height. It isn't the full window height because
that includes the status bar and the minibuffer, and it's annoying to
have to scroll down to see the last few stories.

hackernews.el

@@ -75,7 +75,8 @@
   'hackernews-items-per-page "0.4.0")
 
 (defcustom hackernews-items-per-page 20
-  "Default number of stories to retrieve in one go."
+  "Default number of stories to retrieve in one go.
+If nil, the stories will fill the window."
   :package-version '(hackernews . "0.4.0")
   :group 'hackernews
   :type 'integer)
@@ -471,6 +472,11 @@ Objects are decoded as alists and arrays as vectors.")
       (aset items i (hackernews--read-contents
                      (hackernews--item-url (aref ids (+ offset i))))))))
 
+(defun hackernews--calculate-story-count ()
+  "Return hackernews-items-per-page or enough to fill the window."
+  (or hackernews-items-per-page
+      (- (window-height) 3)))
+
 (defun hackernews--load-stories (feed n &optional append)
   "Retrieve and render at most N items from FEED.
 Create and setup corresponding hackernews buffer if necessary.
@@ -507,7 +513,7 @@ rendered at the end of the hackernews buffer."
                                   (max 0 (min (- (length ids) offset)
                                               (if n
                                                   (prefix-numeric-value n)
-                                                hackernews-items-per-page)))
+                                                (hackernews--calculate-story-count))))
                                   ()))
 
       (hackernews--retrieve-items)