Linter: Verify that keys in the config uses snake_casing

[ventrec]

Valid:

<?php

return [
    'pretend' => env('SMS_PRETEND', false),

    'nexmo_api_key' => env('NEXMO_API_KEY', ''),
    'nexmo_api_secret' => env('NEXMO_API_SECRET', ''),
    'nexmo_sender' => env('NEXMO_SENDER', ''),
];

Invalid:

<?php

return [
    'pretend' => env('SMS_PRETEND', false),

    'nexmoApiKey' => env('NEXMO_API_KEY', ''),
    'nexmoApiSecret' => env('NEXMO_API_SECRET', ''),
    'nexmoSender' => env('NEXMO_SENDER', ''),
];

Should also validate nested arrays in a config file. Needs tests.