Load new helpers from helpers.js.

Custom [Handlebars helpers](http://handlebarsjs.com/block_helpers.html) can be defined in `/helpers.js`. This file needs to export an object defining new helpers, or existing helpers to replace.

```
module.exports = {
  // New helper
  frenchify: function(string) {
    return 'Le ' + string + ' sacrebleu !';
  },

  // Replaces handlebars-helper's uppercase helper
  uppercase: function(string) {
    return string + ' is uppercase';
  },
};
```