Allow batching of partials through folders

Read passed in directories for useable files and turn them into useable partials.

Gulpfile:
````javascript
...
.pipe(handlebars({}, {
    batch: ['templates/layout', 'templates/partials']
}))
...
````

Will register all found files as `parentDir/fileNameWithoutExtension`.

Some templatefile:
````html
{{> layout/header}}
...
{{> partials/bottom_menu}}
{{> layout/footer}}
````

Can probably be improved for broader usage though, but this suits my needs and works.
If this doesn't fit into what you had in mind for your module, I'll try using my fork and maybe publish it as a separate module all together.