Allow modulePrefix customization via config

I customized my wrapper in my config.coffee for brunch. It was no longer CommonJS but since it also wasn't `false`, ember-handlebars-brunch kept its default module prefix of `module.exports = `.

This allows you to change this behavior in the `templates` section of your config.coffee:

```coffeescript
files:
  templates:
    modulesPrefix: false # @modulesPrefix = ''
```
-

```coffeescript
files:
  templates:
    modulesPrefix: 'var template =  ' # @modulesPrefix = 'var template =  '
```