Add a CSRF middleware config variable

[MatanYadaev]

As explained in #53 , Laravel Airlock is blocking the usage of a custom VerifyCsrfToken middleware.
So I added a csrfMiddleware config variable.

I'm very bad with the description stuff, can you guys help me with that? ☺️

[driesvints]

Maybe it's just best to check if the local class exists and use that one instead of a config option?

$verifyCsrfToken = class_exists(\App\Http\Middleware\VerifyCsrfToken::class)
    ? \App\Http\Middleware\VerifyCsrfToken::class
    : \Illuminate\Foundation\Http\Middleware\VerifyCsrfToken::class;

[MatanYadaev]

@driesvints I thought about it, but it won't cover cases that this class has a different name.
I guess it's a very edge-case, so maybe it's a better solution, for saving the package as elegant as it is now.