Allow custom success status codes

[emilsundberg]

Sometimes it might be useful to allow custom http status codes such as 401 which might be a password protected website that is still online.

This PR allows us to configure additional allowed http status codes:

'uptime_check' => [
        // ...
        'additional_status_codes' => [401],
]

Please let me know if this is useful for this package and I'll try to figure out how to add tests for this behaviour.

[riasvdv]

Could you add a test for this?

[emilsundberg]

@riasvdv I've added tests and updated the example in the readme as well.

[riasvdv]

Thanks!