Add support for configuration prefixes

This pull request gives the option to put the configuration data on an arbitrary key of ```$app``` (say, ```$app['config']``` or ```$app['data']```), instead of loading the data directly to ```$app```.

An extra optional parameter was added to ```ConfigServiceProvider```'s constructor to pass the prefixes.

It doesn't support nested prefixes (ie: ``['config']['dev']``). But ```config.dev``` and similar can be used instead.