Fix error when plugin don't extend book

Author: SamyPesse

lib/generate/plugin.js

@@ -163,6 +163,7 @@ Plugin.fromList = function(names, root) {
     resources.html = {}
 
     _.each(plugins, function(plugin) {
+        if (!plugin.infos.book || !plugin.infos.book.html) return;
         var html = plugin.infos.book.html || {};
         _.each(html, function(code, key) {
             if (!_.isFunction(code)) code = _.constant(code);