Don't fail if hook returns nothing for "page" and "page:before"

lib/book.js

@@ -512,7 +512,7 @@ Book.prototype.parsePage = function(filename, options) {
     var interpolate = function(fn) {
         return Q(fn(page))
         .then(function(_page) {
-            page = _page;
+            page = _page || page;
         });
     };