Enable use of {{partial}} helper

By default, Brunch [ignores files beginning with an underscore](https://github.com/brunch/brunch/blob/master/docs/config.md#conventions. This prevents us from using the `{{partial}}` helper as partial templates are required to [begin with an underscore](http://emberjs.com/guides/templates/rendering-with-helpers/#toc_the-code-partial-code-helper).

This configuration change allows us to use templates such as `_user.hbs` and the `{{partial}}` helper.