Add support for passing in custom options

[ctooley21]

Howdy,

My team is currently developing with Checkr, we are currently using their staging environment. Would it be possible to add support in the constructor for overriding the default options? This would be useful if users want to use the staging environment which uses a different baseUrl.

I'll work on creating a PR for this over the next few days. Thanks for creating a really well-developed package! Saved my team and I a lot of time.