Add additional verification method to the Postmark provider

[fsylum]

This PR will add additional verification methods to the Postmark provider.

Summary of changes:

• README update with Configuration section.
• Two new verification methods added.
  • auth will retain the existing functionality.
  • headers will validate against configured header in Postmark webhook page.
  • ips will validate against the provided Postmark IP address on https://postmarkapp.com/support/article/800-ips-for-firewalls#webhooks
• Added related test cases.

Fixes #7.

Please let me know what you think, thanks!

[hotmeteor]

@fsylum Thanks for your contribution! I think this is great.

My only blocker on this is the config. I've tried to make it so that Receiver uses the existing services.php config, just like the "Notifications" feature, or packages like Socialite.

So what I'd prefer is for it not to include a new config file but to instead look for these values in services.postmark. Then we should add some provider-specific configuration settings to the documentation.

Hope that makes sense!

[fsylum]

Thanks @hotmeteor for the feedbacks!

I've removed the custom config file and updated the documentation section for the Configuration part.

Let me know what you think.