Adding a partialsPath option

The default nomenclature for partials – prefixing them with an underscore – works well for small to medium sized projects. But for larger codebases it becomes an issue of practicality to organize types of templates together. For instance, a common pattern is to have a setup like:

```
---templates/
 |--layouts/
 |--partials/
   |--menu.hbs
   |--menu_item.hbs
 |--template_1.hbs
 |--template_2.hbs
 |--etc.
```

This PR adds an option called `partialsPath` which in this example would be set to `templates/partials/`. Used in conjunction with `partialRegex`, an author can configure the task to assume every file in that directory will be a partial.

The default behavior is to leave the path unspecified which allows for partials to live alongside templates.