Fixed reading options from Configuration file

Depends on my enviroment i require different config files with different mustache helpers. Currently the only way to setup the mustache configs is doing it like this:

~~~
$app->register(new MustacheServiceProvider(),array(
    'mustache.options'=>array(
        'helpers'=>array(
            'baseUri'=>'/foobar/'
        )
    )
));
~~~

by setting mustache.options inside a configuration file does not affect the service provider. this small workaround fix the issue