Allow a simple mappings when using Config.

Currently, for simple mappings (where you only specify `class`) you have to do something like this:

``` php
<?php
return [
    'foo' => [
        'class' => 'Dan\Foo',
    ],
    'request' => [
        'class' => function () {
            if (php_sapi_name() === 'cli') {
                return new CliRequest;
            }
            return new WebRequest,
    ],
];
```

With these changes, you can change that to:

``` php
<?php
return [
    'foo' => 'Dan\Foo',
    'request' => function () {
        if (php_sapi_name() === 'cli') {
            return new CliRequest;
        }
        return new WebRequest,
    },
];
```

Much easier.