Load custom Handlebar helpers located in "helpers" dir

Custom Handlebar helpers is a handy way of customizing the kss-node output.

It would be nice if there was some simple built-in way of loading these, like placing files in a given folder

It could be something like the attached pull request, that would read files from a "helpers" directory, given that they were written something like this:

**helpers/foo.js**

```
module.exports.register = function (Handlebars, options)  { 
    Handlebars.registerHelper('foo', function (str)  { 
        return 'foo';
    });
};
```