Rename configuration options

[dwightwatson]

We currently use app_id and app_secret as configuration option names, which does make sense as they are the names Facebook uses, but I wonder if perhaps we'd be best to match the naming scheme of the Socialite channels.

Here's how it would look right now if you used Socialite and this package together, which I think just looks a little odd and inconsistent.

    'facebook' => [
        'client_id' => env('FACEBOOK_KEY'),
        'client_secret' => env('FACEBOOK_SECRET'),
        'redirect' => '/verifications/facebook/callback',
    ],

    'facebook_poster' => [
        'app_id' => env('FACEBOOK_KEY'),
        'app_secret' => env('FACEBOOK_SECRET'),
        'access_token' => env('FACEBOOK_ACCESS_TOKEN'),
    ],