Add support for custom handlebars templates & vars

This commit fixes gh-1:

Load automatically `helpers.js` if present in directory. 
Add `--helpers` command line option to load a different helpers definition file.

Helpers definition file example:

```javascript
module.exports = function(Handlebars) {
  Handlebars.registerHelper('date', (function() {
    var date = new Date();
    return function(options) {
      return date.toString();
    };
  })());
};
```

And, example of use:

```
# Generation Date : {{date}}
```