Optional configuration without environment variables

[trusche]

Hi @coorasse,

we just started using your gem in our Rails app, it's working great, so thank you, first of all!

We ran into a bit of trouble deploying this, though. If any of the required env variables is not set, not only will the gem not work (expected and acceptable), but the rails app will fail to start (not so good 😁 ).

This PR adds a couple of configuration changes, and might help addressing both #34 and #5.

• A configuration option for the check on required configuration attributes (defaults to true, so no behaviour change on upgrade, it must be set to false explicitly in an initializer)
• Replaced the use of all env variables with the equivalent configuration option - they're already assigned from env vars on startup, so might as well use them. This allows users to configure the gem completely from within an initalizer, with env variables or other mechanisms like Rails secrets.

Tests are still passing, but I didn't add any new ones since I'm more familiar with rspec and a bit short on time right now. Happy to amend that sometime later if needed.

Cheers,
Thilo

[trusche]

I've thrown that in here since we use it in our app to conditionally display passkit-related view code only if the gem is properly configured.

[trusche]

Constants are evaluated at load time, not runtime, so if those envs are nil, this will raise an error at startup of a rails app with eager_load = true. This bit us in production no less, because in staging we had everything configured correctly, and in local environment classes are not eager loaded, so it didn't manifest there.