IPinfo URL could be overwritable

[underq]

Hi,

During the development stage I used lot of mock to preserve the API credits, but it's impossible with ipinfo/php library because the const API_URL is really hard coded.

There are lot of possibility for render the usage more flexible, use static::API_URL instead of self::API_URL or add $api_url as a constructor param.

I can open a PR if you think that is a good evolution

Best regards