fix:Add utcOffset to handlebars dateformat helper

Tests fail because the dateFormat helper uses the local timezone for
formatting the date. This results in unexpected dates, if the local time
is still in the day before or already in the day after, compared to the
UTC date in the source.

test/index.js

@@ -9,7 +9,7 @@ var slug = require('slug');
 
 Handlebars.registerHelper('dateFormat', function(context, format) {
   var f = format || 'DD/MM/YYYY';
-  return moment(new Date(context)).format(f);
+  return moment(new Date(context)).utcOffset('+0000').format(f);
 });
 
 describe('metalsmith-tags', function() {