Allow Multiple Config Classes for Each Type

So something like this might be in a `composer.json`:

    {
        "extra": { "aura": {
            "type": "project",
            "config": {
                "common": [
                    "Acme\\Example\\_Config\\AuraSql",
                    "Acme\\Example\\_Config\\Twig"
                ],
                "dev": "Acme\\Example\\_Config\\Dev"
            }
        } }
    }

A bigger application might want to separate configuration into several
different classes. Normal strings still work, but arrays may be used as
well.
Handle Strings Explicitly, Cast to Array

Makes it a bit easier to handle objects vs arrays in the JSON.
Merge pull request #8 from chrisguitarguy/multiple_configs

Allow Multiple Config Classes for Each Type