Add defaults filters to website tpl engine

lib/output/website.js

@@ -6,6 +6,7 @@ var I18n = require('i18n-t');
 
 var Promise = require('../utils/promise');
 var fs = require('../utils/fs');
+var defaultFilters = require('../template/filters');
 var conrefsLoader = require('./conrefs');
 var Output = require('./base');
 
@@ -89,6 +90,12 @@ WebsiteOutput.prototype.prepare = function() {
 
         that.env = new nunjucks.Environment(new nunjucks.FileSystemLoader(_.map(searchPaths, templatesPath)));
 
+        // Add GitBook default filters
+        _.each(defaultFilters, function(fn, filter) {
+            that.env.addFilter(filter, fn);
+        });
+
+        // Translate using _i18n locales
         that.env.addFilter('t', function(s) {
             return that.i18n.t(that.book.config.get('language'), s);
         });